Process for the dewatering of coal and mineral slurries

Coal or other mineral slurry is dewatered by establishing a bed of the slurry and injecting a gas stream such as air into the bed to establish turbulent flow to strip moisture. This slurry may use particles in the range 0.5 mm to 30 mm with air injected at about 10 m/sec, and suitable novel apparatus includes a centrifuge or a vibratory conveyor with a closed tunnel and transverse air flow for stripping moisture.

FIELD OF THE INVENTION 
The present invention relates to an apparatus and method for reducing 
moisture content of a particulate mass such as ground mineral material 
e.g. coal. Typically minerals and especially coal products contain a 
substantial percentage by weight of water and accounting for as much as 
10% of the mass. In this specification, particular attention will be given 
to the treatment of coal, but it is to be understood that apparatus 
embodying the invention and the methods of the invention may also be 
applicable to other similar mineral masses which in their initial state 
are described as slurries. 
BACKGROUND TO THE INVENTION 
Conventional processes for moisture removal from minerals such as coal 
include screening, centrifugation and vacuum filtration. In the case of 
coal products, it is economically important to reduce the moisture content 
prior to land transport of the particulate mass as transport costs are 
essentially according to weight and it is an economic penalty to transport 
as much as 10% of the weight of the product as unwanted water. 
Furthermore, in some industrial processes using coal products, such as 
power generation, it is a substantial thermal penalty to have a 
significant level of moisture in the coal as energy in burning the coal is 
then utilised in driving off the moisture as steam. 
For many years, it has been conventional to use centrifuges to reduce 
moisture levels to around 6 to 8 wt % where the particle size range is 
typically less than 30 mm and greater than 0.5 mm. With conventional 
practice, a practical limit for dewatering is controlled by the relative 
opposing magnitudes of capillary forces causing the water to be retained 
in the particulate mass and the applied forces attempting to strip the 
water from the mass. 
SUMMARY OF THE INVENTION 
In a method aspect, broadly the present invention consists in a method of 
reducing moisture content of a bed of solid particles comprising 
subjecting the bed to a stream of gas to establish turbulent flow through 
the bed to strip a significant proportion of the moisture contained in the 
bed. 
In an apparatus aspect, the present invention consists in an apparatus for 
processing a bed of solid particles containing moisture, the apparatus 
comprising a processing zone for receiving the bed, means for admitting 
and injecting into the bed a gas stream so as to establish a turbulent 
flow through the bed and to strip moisture, discharge means being provided 
for discharging the gas with entrained moisture. 
A most important embodiment of the invention is one in which processing of 
the bed takes place in a centrifuge which for a practical commercial 
embodiment would be a continuously operating centrifuge. However other 
embodiments are possible such as advancing the particulate solids in the 
form of a bed which is moved along a vibratory conveyor such as downwardly 
inclined tunnel containing a processing zone in which the gas is injected 
to strip moisture. 
It is believed the present invention can successfully reduce the residual 
moisture in a mineral such as coal and it is considered that a significant 
advantage can be achieved by reducing the moisture level by 1 wt % of the 
mass over and above that achievable by known methods such as 
centrifugation. While not being bound by any particular theory, as can aid 
to understanding the present invention, the inventors suggest that useful 
results of the present invention are due to enhanced kinetics resulting 
from a mass transport mechanism brought about by the superimposition of a 
turbulent gas flow through the bed. 
Preferably, the invention is operated with particles in the range of mainly 
0.5 mm to 30 mm although it is acceptable to have a proportion of the 
particles outside this range. The invention has been found to operate 
advantageously where 90% of the particles in the mass have a size greater 
than 1.5 mm and the particle distribution is such that a very low level of 
fines i.e. less than 0.5 mm are present whereby turbulent gas flow can 
readily be sustained. It is thought that it is in the turbulent flow which 
entraps free moisture and removes it. 
By contrast, prior published proposals do not include other than using air 
flow with very fine coal particles and wherein laminar flow conditions 
were applied. 
Preferably the present invention is implemented using a relatively low 
pressure air flow as the turbulent gas and this is believed to be 
particularly successful in promoting hydrodynamic drag of liquid from 
within the inter-particle voids. 
The present invention is believed to be particularly applicable to 
particles having a strong hydrophobic characteristic. It has been found 
that coal has such a characteristic but other minerals also share this 
feature. Another application of the invention is one where the method 
comprises preliminary treatment of particulate matter with a compound to 
provide a surface effect on the particles whereby a substantial 
hydrophobic characteristic is established. Then the material can be 
successfully processed according to principles of the present invention. 
For convenience and economy, air has been found to be an effective medium 
for the turbulent gas flow. The air can be at ambient temperature. However 
other gas flows can be used such as steam and other gases of elevated 
temperature. 
The speed of air flow passing through the particulate mass can be 
conveniently chosen and in general, it has been found that a speed in the 
range of 1 to 20 m/sec is beneficial and preferably around 10 m/sec offers 
a convenient and economic choice. 
The invention can be implemented by adaptation of known types of 
centrifuges of which a vibrating basket type continuous centrifuge is 
particularly attractive for commercial operations. Preferably a vibrating 
basket centrifuge is used with a novel air inlet manifold provided to 
inject air at a multiplicity of locations spaced from and around the axis 
of the basket. Air can be injected through a manifold having a series of 
short pipes substantially parallel to the axis of the basket and having 
apertures for directing air jets radially outwardly. 
However, other types of centrifuge could be used such as scroll and screen 
bowl centrifuges. 
Particularly, when a vibrating basket centrifuge is used, operation at a G 
force in a range 25 G to 120 G is suitable with basket speeds in the range 
of 200 to 450 rpm.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, a centrifuge basket 10 is mounted on a rotary bearing 
11 drilled through the centre to provide an air inlet 12 leading to a 
chamber 13 from which radially outwardly bores 14 pass to an outer chamber 
15. A batch of particulate coal is located in an annular basket 16. 
The illustrated centrifuge is for laboratory scale batch operations and has 
been used to test out the principles of the invention which will be 
described further below with reference to data derived from testing. As it 
was not possible to measure air speed while the centrifuge was spinning, 
an anemometer was used on the outside of the stationary basket packed with 
coal before starting centrifuge operations in order to measure air 
velocity through the coal bed. 
Referring now to FIG. 2, a more practical continuous centrifuge is 
illustrated. This is a scroll centrifuge of known type but modified for 
the introduction of pressurised gas such as air or steam to implement the 
concepts of the present invention. In this centrifuge, 20, there is a cone 
21 mounted on a rotor 22 and the cone carrying a series of flights 23 down 
which the coal mass progressively moves to annular discharge location 24. 
Coal is fed into the centrifuge through an upper axial inlet 25. The rotor 
is mounted on a hollow drive shaft 26 connected to an air pressure line 
through an air seal 27 whereby pressurized air is introduced into the cone 
from which it is radially outwardly discharged through apertures 28 in the 
cone. 
Referring to FIG. 3, data are presented for coal particles ranging from 0.5 
mm to 9.5 mm which were subject to centrifuging. Curve 30 represents wet 
coal with no air purge, curve 31 represents air dried coal treated without 
air purge and curves 32 and 33 are for wet coal and air dried respectively 
with air injected at 10 m/sec for a purge time of 10 seconds during the 
centrifuge operation in order to strip moisture. Resulting residual 
moisture level in the coal bed is indicated for different G force values 
corresponding with different centrifuge basket speeds. The results 
indicate a substantial improvement in reducing moisture level when 
contrasting data for use of the air purge with the centrifuging as opposed 
to centrifuging alone without the air purge. 
In each case the initial moisture content was about 10 wt %. 
Referring now to FIG. 4, the plot of the rate of moisture loss with varying 
air speed shows a marked change in the rate of moisture loss corresponding 
to gas flow velocities above about 1 m/sec. This indicates a change of 
mechanism from evaporation at low flow rates to bulk mass transport. 
FIG. 5 demonstrates that steam is an alternative to air and significant 
moisture reduction can be achieved according to this experimental data. 
FIG. 6 illustrates the data to show a typical profile for moisture 
reduction plotted against air velocity. Thus it will be seen that with 
coal particles with a size range typically 1 mm to 10 mm in the main, 12 
m/sec is an effective and economically feasible air flow velocity to be 
utilised. 
FIG. 7 illustrates an experiment on fine coal particles in the range below 
3.35 mm but greater than 0.5 mm using an air speed of 10 m/sec and purge 
time of 10 sec. The contrasting data of using an air purge as against 
merely centrifuging shows a substantial reduction in moisture with, 
particularly in the case of air purge, only a small improvement when 
increasing centrifuge speed to correspond with an increase in G force from 
50 G to 200 G. 
FIG. 8 corresponds to FIG. 7 data but uses relatively coarse coal particles 
in the size range below 9.5 mm and above 3.35 mm. 
The above data demonstrates the principles of the invention can be 
effectively applied to a range of particulate sizes. Reference will now be 
made to FIG. 9 illustrating various states in which water is thought to be 
present in a particulate bed of coal particles. In the saturated state 
(FIG. 9A), water is held under capillary forces to fill the 
inter-particulate voids. In the pendular state (FIG. 9C), moisture is 
retained at points of contact between individual coal particles but there 
is believed to be an intermediate state referred to as the funicular state 
(9B) in which moisture exists in equilibrium with air dispersed throughout 
the porous structure. It is suggested that by normal centrifugation of 
typically coal products (which have not been air dried) there is a limit 
to the level to which free moisture can be reduced and this is determined 
primarily by the amount of pendular moisture which, depending on the mode 
of packing, can be shown theoretically to be around 5 to 7 wt % for a 
wetting liquid. This figure is in fact consistent with measured values for 
residual moisture from reported commercial coarse coal centrifuge 
processes. FIGS. 7 and 8 provide data contrasting fine and coarse coal 
particle masses but otherwise processed under similar conditions. The 
residual moisture levels are considerably higher with the fine coal 
fractions but the moisture reduction achieved by the combination of air 
purge and centrifugation was considerably greater for the finer fractions 
at all levels of spin speed. Thus at a speed equivalent to 50 G, a 
reduction in moisture achieved for fine particles was about 3 wt % 
compared with about 1 wt % for the coarse particles. 
Without being bound to any particular theory the present inventors suggest 
this data may show two possible phenomena occurring. It is suggested that 
for the finer coal particles there will be a greater amount of pendular 
moisture present and which will be available for displacement by the air 
purge during centrifugation. Secondly the finer the size of the coal 
particles, the finer will be the size of the inter-particle pores within 
the bed. This in turn should lead to an increase in turbulence as the air 
purge occurs and the inventors suggest that this greater turbulence and a 
thinner boundary layer would make the air purge more effective at removing 
water. Accordingly, when a complete sized distribution of coal particles 
is used (say less than 9.5 mm and above 0.5 mm) dewatering characteristics 
can be achieved more akin to fine coal particles rather than coarse coal 
particles due to turbulence within pores of the structure. A particulate 
batch of coal particles mainly in the range of 1 mm to 10 mm is believed 
to have a greater amount of moisture present in the pendular state. 
Referring now to the embodiment of FIG. 10, which is a vibratory conveyor 
system, the apparatus comprises a shute 40 having an inlet hopper 41 for 
receiving particulate coal and a lower discharge port 42, the shute being 
mounted on a vibratory feeder 43 which causes steady advance of the 
particulate matter in the form of a bed. In its upper mid-portion, the 
shute has a manifold 44 connected to a compressed air supply line 45 which 
discharges a band of air downwardly through the bed for discharge through 
a suitable grating (not shown in the drawing) covering an air outlet 46. 
The air is supplied at such flow rate and pressure having regard to the 
particle sizes in the bed so that turbulent air stream establishes through 
the bed whereby moisture and in particular moisture in a pendular state is 
stripped from the bed. 
Referring now to FIGS. 11 and 12, this embodiment has a novel manifold 
arrangement applied to a vibrating basket centrifuge 50. The centrifuge 
comprises a frusto-conical basket 51 having an end wall 52 and at its 
opposite end an air manifold 53 comprising a part circular tube having 
ports 54 at each (and for the introduction of pressurised air and lateral 
air discharge tubes 55 each having a series of apertures for directing air 
jets generally radially outwardly. As shown in FIG. 11 pressurised air is 
fed through line 56 to each of the ports 54. Particulate coal or other 
mineral is supplied into the basket through a tubular duct 57 which 
discharges the particulate coal adjacent the wall 52. The basket is 
rotated and vibrated horizontally and dried, treated coal particles are 
discharged at the bottom of the basket as indicated by arrow A into a 
receiving hopper 58. 
In this apparatus the coal particles move under the influence of the 
vibrations to the wider open end of the basket where discharge takes 
place. This apparatus is suitable for use in dewatering coal particles 
with particle sizes in the range of 30 to 0.5 mm.