Thesis (MScEng)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Ferrohydrostatic separation (FHS) of materials is a float and sink
technique which utilizes ferrofluid exposed to a non-homogeneous
magnetic field. The efficiency of material separation depends on
numerous variables. The most important variables, which were
investigated individually, are the effects of moisture content, ferrofluid
level, feedrate, particle size and material density distribution on separation
efficiency.
It is important to recover and recycle the ferrofluid attached to the
products of separation so as to reduce the cost of the FHS technology and
the amount of kerosene disposed of to the environment. This prompted
research into some of the factors affecting ferrofluid recovery. The factors
that were investigated are the effects ofFHS operation, material moisture
content, particle size and porosity.
The separation efficiency was found to be dependent on all the variables
investigated. The effect of material moisture content is less pronounced
for particles larger than 2.8 mm. This implies that wet feed material
should be screened before ferrohydrostatic separation and material which
particle size is less than 2.8 mm should preferably be treated dry. Wet
material (less than 2.8 mm) floats, even though its density is greater than
the cut-point density. This is owing to the immiscibility of the water
coating the particles and the kerosene-based ferrofluid used for separation.
It was found that the effect of ferrofluid level on separation efficiency is a
function of both the density difference of the particles to be separated and
the particle size. Separation efficiency as a function of ferrofluid level is
poor for particles larger than 2 mm and is good when the density
difference of the material to be separated is high, for instance between
2700 kg/nr' and 3530 kg/nr'. This shows that for efficient separation of
coarse particles and near density material (material with density close to
the cut-point density), the ferrofluid level should be controlled, preferably
close to the maximum possible level.
The effect of feedrate on separation efficiency is also a function of the
densities of the particles to be separated. An increase in feedrate leads to
poor separation for particles with densities close t~ each other. This
implies that separation of near density material requires accurate feedrate
control. It has been shown from simulation and modelling that the effective cutpoint
density changes as the particle moves through the chamber until it
eventually reaches its terminal velocity, given that the chamber is of
sufficient size for this to occur. The effective cut-point density increases to
the maximum as the particle enters the ferrofluid pool but settles down to
a relative constant once the particle has reached its terminal velocity. The
effective cut-point density was shown to decrease with an increase in
particle magnetisation. It was found that this decrease in the cut-point
density determines the density difference (difference between two
particles) achievable when non-magnetic material is treated together with
magnetic material. It is therefore important to magnetically scalp the feed
material for efficient separation. When the material is not scalped,
magnetic and nonmagnetic material with the same density might report to
different density fractions, which leads to poor separation. This magnetic
contribution to the effective density can be utilised in the separation of
material with same density but different magnetisation.
The efficiency offerrofluid recovery was found to be dependent on all the
variables investigated. The amount of ferrofluid drawn from the FHS
separator was found to decrease with an increase in the magnetic field.
Furthermore, the amount of ferrofluid that remains attached to the
particles after allowing ferrofluid to drain from the material is the same as
that attached to the FHS products of separation at high magnetic fields.
This shows that it is important to operate the ferrohydrostatic separator at
high magnetic fields in order to attract most of the ferrofluid back to the
separator.
T-heamount of ferrofluid adsorbed onto and absorbed by the particles was
found to decrease with an increase in the material moisture content. This
is due to two factors. The first is that water occupies the vacant pores in
the material. The second is that water forms a layer on the particle surface
which is immiscible with kerosene-based ferrofluid. This phenomenon
leads to a reduction in cost of the ferrohydrostatic separation technology
when wet material as opposed to dry material is treated. As already
described coarse material larger than 2.8 mm can be treated wet without
detrimental effects on separation. For -8+4 mm particles, the ferrofluid
loss ranges from 0.6 down to 0.14 kg/tonne of feed for 0 to 10 % material
moisture content respectively.
The amount of ferrofluid lost per tonne of feed was found to range from
0.73 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 in
surface area in small particles for ferrofluid adsorption.
The increase in porosity increases the amount of ferrofluid lost due to the
difficulties in recovering ferrofluid embedded in the pores of the particles. Adding water to coarse material lowers the amount of ferrofluid lost by
reducing porosity. Modelling the amount of ferrofluid lost, as a function
of particle size and porosity, would assist in determining the amount of
ferrofluid required to treat a known amount of material.
The quality of ferrofluid recovered was found to be the same as that
initially used for material separation. This implies that the separation
efficiency would not be affected by the use of recycled ferrofluid. / AFRIKAANSE OPSOMMING: Ferro-hidrostatiese skeiding van materiale is 'n flotasie (dryf) en
besinkingstegniek wat gebruik maak van ferro-vloeistof wat blootgestel is
aan 'n magnetiese veld. Die effektiwiteit van die materiaal skeiding is
afhanklik van verskeie veranderlikes. Die belangrikste veranderlikes wat
die skeidingseffektiwiteit beïnvloed is individueel bestudeer, naamlik
voginhoud, ferro-vloeistof vlak, voertempo, partikelgrootte en materiaal
digtheid verspreiding.
Dit is belangrik om die ferro-vloeistof te herwin en te hergebruik om die
koste van die proses en tegnologie te verminder en dus ook die
hoeveelheid keroseen wat aan die omgewing blootgestel is. Dit het
navorsing tot gevolg gehad oor die faktore wat ferro-vloeistofherwinning
beïnvloed. Hierdie.faktore wat ondersoek is in hierdie studie is materiaal
voginhoud, partikelgrootte en porositeit.
Die skeidingseffektiwiteit was afhanklik van al die faktore wat ondersoek
is. Die effek van materiaal voginhoud was minder beduidend vir partikels
groter as 2.8 mm. Dit wys dat nat voermateriaal moet gesif word voor
ferro-hidrostatiese skeiding, en materiaal met 'n partikelgrootte kleiner as
2.8 mm moet verkieslik gedroog word. Nat materiaal (minder as 2.8 mm)
floteer selfs as die digtheid groter is as die snypunt digtheid. Dit is as
gevolg van ondeurlaatbaarheid van die water wat die partikels bedek en
die keroseen basis ferro-vloeistofwat gebruik word vir die skeiding.
Dit is gevind dat die invloed op die skeidingseffektiwiteit van die ferrovloeistof
vlak is 'n funksie van beide die digtheid van die partikels wat
geskei word. Die partikelgrootte skeidingseffektiwiteit as 'n funksie van
die ferro-vloeistof vlak, is swak vir partikels groter as 2 mm en is goed
wanneer die digtheid verskil van die materiaal wat geskei moet word,
hoog is, byvoorbeeld 2 700 kg/nr' en 3 530 kg/nr'. Dit wys dat vir die
effektiewe skeidings vir groter partikels en naby digtheid materiaal
(materiaal net 'n digtheid nabyaan die snypunt digtheid), moet die ferro- .
vloeistofvlak baie goed beheer word, gewoonlik naby die maksimum vlak
moontlik.
Die effek van voertempo op die effektitiwiteit van skeiding is ook 'n
funksie van die digtheid van die partikels wat geskei moet word. 'n
Toename in die vloeitempo lei tot 'n swak skeiding van partikels met
digthede wat naby mekaar lê. Dit wys weer daarop dat die skeiding van
naby digtheid materiaal het die akkurate beheer van voertempo tot gevolg. Dit is gevind deur simulasie en modulering dat die effektiewe
snypuntdigtheid verander soos die partikel deur die kamer beweeg totdat
dit uiteindelik sy finale snelheid bereik (gegee dat die kamer groot genoeg
is). Dit effektiewe snypunt digtheid verhoog tot 'n maksimum wanneer
die partikel die ferro-vloeistof binne gaan, maar bereik na 'n kort tydperk
'n kostante waarde sodra die partikel sy finale snelheid bereik het. Die
effektiewe snypunt digtheid verlaag met 'n toename in partikel
magnetisme. Dit is gevind dat die afname in die snypunt digtheid bepaal
die digtheidsverskil (verskil tussen twee partikels) wat bereikbaar is
wanneer nie-magnetiese materiaal saam met magnetiese materiaal
behandel word. Dit is dus belangrik om die voer materiaal magneties te
skalpeer vir effektiewe skeiding. Wanneer die materiaal so behandel
word, sal magnetiese en nie-magnetiese materiaal, met die dieselfde
digthede, rapporteer in verskillende digtheidsfraksies wat sal lei tot swak
skeiding. Die magnetiese bydrae tot die effektiewe digtheid kan gebruik
word in die skeiding van materiaal met dieselfde digtheid, maar met
verskillende magnetismes.
Die effektiwiteit van ferro-vloeistof herwinning is afhanklik van al die
veranderlikes wat ondersoek is. Die hoeveelheid ferro-vloeistof wat
omtrek is van die ferro-hidrostatiese skeier verminder met 'n toename in
die magnetiese veld. Verder is die ferro-vloeistof wat agterbly as gevolg
van die feit dat hulle vas is aan die partikels na dreinering, dieselfde as die
hoeveelheid wat vasgeheg is aan die die ferro-hidrostatisiese produkte van
skeiding by hoë magnetiese velde. Dus is dit belangrik om die ferrohidrostatiese
skeier te bedryf by hoë magnetiese velde om sodoende die
meerderheid van die ferro-vloeistof in die skeier agter te laat bly.
Die hoeveelheid ferro-vloeistof geadsorbeer aan en geabsorbeer deur die
partikels verlaag met 'n toename in die materiaal voginhoud. Dit is
gevolg van twee redes, nl. eerstens water wat die plek inneem van die oop
porieë in die materiaal, en tweede is die feit dat water 'n lagie op die
partikeloppervlakte vorm wat ondeurlaatbaar is vir keroseen-basis ferrovloeistof
Dit lei tot die vermindering in koste van die ferro-hidrostatiese
skeidingstegnologie wanneer nat materiaal in plaas van droë materiaal
gebruik word. Soos alreeds genoem, partikels groter as 2.8 mm kan nat
behandel word sonder enige negatiewe effekte op die skeiding. Vir -8+4
mm partikels is daar 'n ferro-vloeistofverlies van 0.6 tot 0.14 kg/ton voer
vir' n 0-10% voginhoud.
Die hoeveelheid ferro-vloeistof per ton voer materiaal wat verlore gaan
wissel tussen 0.73 tot 0.56 kg vir -0.85+0.5 mm tot -12+8 mm
partikelgroottes, respektiewelik. Die toename in ferro-vloeistofverlies by
kleiner partikels is as gevolg van die toename in die oppervlakarea van
kleinpartikels vir ferro-vloeistof adsorpsie. Daar is 'n toename in porositeit wat gepaard gaan met 'n toename in
hoeveelheid in ferro-vloeistof wat verlore gaan as gevolg van probleme
met die herwinning van ferro-vloeistof wat binne-in partikelporieë
vasgevang IS.
Die byvoeging van water by groter materiaal verlaag die hoeveelheid
. ferro-vloeistofwat verlore gaan as gevolg van verminderde porositeit. Die
modellering van die hoeveelheid ferro-vloeistof wat verlore gaan, as 'n
funksie van die partikelgrootte en porositeit, sal help met die skatting van
die hoeveelheid ferro-vloeistof benodig om 'n sekere hoeveelheid
materiaal te behandel.
Dit is gevind dat die kwaliteit van die ferro-vloeistof wat herwin word
dieselfde is as die wat aanvanklik gebruik is vir die materiaal skeiding.
Dit wys dat die skeidingseffektiwiteit beïnvloed nie die gebruik van
gehersirkuleerde ferro-vloeistof nie.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52456 |
| Date | 04 1900 |
| Creators | Dumbu, Stanford |
| Contributors | Svoboda, J., Lorenzen, L., Petersen, K.R.P., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 119 p. |
| Rights | Stellenbosch University |
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