Browsing by Author "Dumbu, Stanford"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Some aspects of ferrohydrostatic separation of minerals and the recycling of ferrofluid
    (Stellenbosch : Stellenbosch University, 2001-04) Dumbu, Stanford; Svoboda, J.; Lorenzen, L.; Petersen, K. R. P.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    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.