DR method of operating a centrifuge filter

A method of operating a drum centrifuge having a foraminous drum rotatable about an axis it is centered on comprises first filling a charge of a suspension into the drum while rotating it about its axis so that the charge forms an annular stratified body having an inner surface and the liquid phase of the body passes radially outward and leaves behind the solid phase as a filter cake and then refilling at least one additional charge of a suspension into the drum onto the filter cake while rotating the drum as in the preceding filling step to add the solid phase of the additional charge to the cake already in the drum. Then at least periodically the radial position of the inner surface of the body in the drum is detected and the dry point when the liquid phase has substantially passed radially out of the drum is ascertained to generate outputs corresponding to the detected radial positions and the times same are detected. The filter cake is then washed by passing a wash liquid therethrough for a time determined by the outputs and thereafter the washed filter cake is centrifuged for a time determined by the outputs.

FIELD OF THE INVENTION 
The present invention relates to a filter-centrifuge system. More 
particularly this invention concerns a method of operating such a system. 
BACKGROUND OF THE INVENTION 
A standard drum centrifuge has a housing in which a foraminous drum is 
rotated at high speed about its axis. A suspension is fed to the interior 
of the drum so that it is thrown centrifugally against the wall thereof. 
At first the suspension forms an annular body in the drum having an inner 
surface centered on the axis, then the body stratifies and the liquid 
phase passes through the drum and the solid phase stays behind on the 
inner surface of the drum as a filter cake. This inner layer of liquid 
passes radially outward through the layer of solids until same is 
substantially dry. As a rule the drum is filled and refilled several times 
until the filter cake builds up to a desired depth. Then this cake is 
washed by passing a liquid through it, and then it is centrifuged to an 
extremely low moisture content. Subsequently a hot gas can be passed 
through it to further dry it, and finally it is physically stripped out of 
the drum, same is regenerated, and the cycle is restarted. 
The level, that is the radial position relative to the drum rotation axis, 
of the inner surface of the annular body formed by the liquid and solid 
fractions can be sensed by a detector such as described in German patent 
document 3,726,227 filed 07 Aug. 1989 by peter Sedlmayer, or by a system 
such as described in patent application 07/614,808 filed 16 Nov. 1990 by 
Rainer Kampschulte. Such sensors can even detect when the liquid has run 
through the cake and the top of the body in the centrifuge is in fact 
formed by solids, the so-called dry point. 
The centrifuging and drying time is fairly long compared to the time 
necessary to spin the liquid fraction out of each batch. Thus for maximum 
efficiency each batch must be as large as possible, capable of filling the 
drum inward to a level just below the inlet. On the other hand the filter 
cake must be reduced to a fairly low residual moisture content. 
As discussed in East German patent 218,283 (D. Trumper), and in West German 
patents 1,036,763 (J. Hertrich), 1,186,411 (K. Zeppenfeld et al), 
2,441,849 (H. Bitus), 2,525,232 (W. Schillig), 2,649,037 (H.Papezik), and 
3,615,013 (P. Franzen) the rate at which the liquid level, distinguished 
from the underlying solids level, drops in the drum is a function of the 
composition of the fractions. The particle size of the solid fraction, 
viscosity of the liquid fraction, thickness of the filter cake, thickness 
of the base layer underlying the cake, and other factors all affect the 
rate at which liquid can be driven out of the suspension being filtered. 
Existing technology does not allow these factors to be taken into account, 
so the refilling time and cycling time are usually set somewhat longer 
than is strictly necessary to produce a filter cake of the desired low 
moisture content, since to err on the side of a too wet product is to 
produce something that will have to be recycled through the drum 
centrifuge before it can be used. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
drum-centrifuge system and method of operating same. 
Another object is the provision of such an improved drum-centrifuge system 
and method of operating same which overcomes the above-given 
disadvantages, that is which produces a filter cake of the desired low 
moisture content in the bare minimum amount of time necessary to do so. 
SUMMARY OF THE INVENTION 
A method of operating a drum centrifuge having a for aminous drum rotatable 
about an axis it is centered on according to this invention comprises 
first filling a charge of a suspension into the drum while rotating it 
about its axis so that the charge forms an annular stratified body having 
an inner surface and the liquid phase of the body passes radially outward 
and leaves behind the solid phase as a filter cake and then refilling at 
least one additional charge of a suspension into the drum onto the filter 
cake while rotating the drum as in the preceding filling step to add the 
solid phase of the additional charge to the cake already in the drum. Then 
at least periodically the radial position of the inner surface of the body 
in the drum is detected and the dry point when the liquid phase has 
substantially passed radially out of the rum is ascertained to generate 
outputs corresponding to the detected radial positions and the times same 
are detected. The filter cake is then washed by passing a wash liquid 
therethrough for a time determined by the outputs and thereafter the 
washed filter cake is centrifuged for a time determined by the outputs. 
With the method according to this invention a sensor such as described in 
the above-identified patent application monitors the level of the body in 
the drum during filtering and washing, with either continuous or periodic 
sampling, so as to determine the change with respect to time of the level 
of the stratified liquid/solids body in the drum. Then the dry points, 
that is the instants when the sensor riding on the annular body in the 
drum is no longer riding on a liquid but on solids because the liquid 
level is below the solids level, are determined. From the change with 
respect of time of he level and the dry points it is possible to determine 
the optimal number of fill cycles, the optimal time to start the wash 
cycle, and the amount of time to centrifuge to produce the desired 
residual moisture content in the filter cake. 
In this manner the fill, liquid-extracting, and washing steps are 
determined independently of how the apparatus is filled so that the 
throughput of the filter can be maximized while producing a uniform end 
product. 
The invention is based on the surprising discovery that all factors 
affecting the filtering, washing, and drying time are seen in the speed at 
which the level changes during filtering and washing. These factors can 
themselves be the products of characteristics such as temperature, 
viscosity, particle, size, practice shape, and numerous other parameters 
of the machinery and of the material being filtered. 
Changes from load to load can be compensated for by different cycle times 
so as to completely avoid producing loads that are too wet and that need 
retreatment. The necessary regeneration steps such as washing out, 
scraping, or replacing the filter medium are also indicated by the level 
change with respect to time and are automatically carried out. The process 
can work continuously and downstream devices like dryers can be used 
optimally.

SPECIFIC DESCRIPTION 
As seen in FIG. 1 a suspension is first filled into a drum centrifuge which 
is then spun at high speed to filter it. The drum is refilled with more 
suspension then and spun again to refilter it, and these two steps are 
repeated as often as necessary to achieve the desired thickness of the 
filter cake. The drum is then spun while a wash liquid is passed through 
the cake to strip all of the remaining liquid fraction from it, and then 
is spun without the addition of more suspension or liquid to dry the 
filter cake. A dry gas (or even drying liquid) may then optionally be 
passed through the filter cake which is thereafter stripped from the drum 
by means of a blade. Subsequently the filter is regenerated by changing 
the filter medium, flushing the stripped drum, or other standard 
procedures. 
FIG. 2 shows in a solid line the depth of the body in the drum, the radial 
thickness h of the body being on the ordinate and time t being plotted on 
the abscissa. The sawtooth or squiggly line shows the increase in the 
thickness of the filter cake and the intersection of the sawtooth and 
solid lines, such as at W and ET, indicate the dry points achieved before 
and after washing. The dashed line shows the level of the liquid phase 
which is normally unimportant after it is below the level of the solids 
phase. 
FIG. 3 schematically illustrates the decrease in level of the wash liquid 
before the centrifuging step. The decreasing height h is measured at 
regularly spaced intervals and is stored so that a microprocessor ca 
derive the differential quotient dh/dt. The entire level goes down, that 
is toward the drum axis, until the level h.sub.ET of the filter cake is 
reached, the so-called dry point at which the liquid has passed through 
the solids and the sensor S (FIG. 5) is resting directly on the filter 
cake. The time t.sub.ET at which this dry point is reached is recorded. At 
this instant the extraction by centrifuging and the centrifuging time 
T.sub.s starts. This time t.sub.s which is the largest part of the overall 
cycle length is therefore determined in accordance with the factors 
H.sub.ET and dh/dt as well as the machine sizes and a constant K 
determined by drum speed. 
The changing filtration characteristics are dependent on the changing 
composition of the suspension being filtered For instance particle shape, 
average particle size (d-p50), the shape and slope of the sum curves of 
the particle-size analysis, proportion of fines, feed concentration, 
liquid temperature and viscosity are determinative. These production 
characteristics are set by the parameters h.sub.ET and dh/dt sufficiently 
accurately. 
In order to reach the desired moisture content the following formula 
pertains: 
EQU tS.apprxeq.{h.sub.ET /h.sub.ETO }.sup.b t.sub.so {(dh/dt).sub.o 
/dh/dt)}.sup.c, 
where: 
K=a constant determined by the centrifuge, 
a, b, c=fixed exponents, 
h.sub.ETO =height of inner surface at dry point for the liquid phase, 
t.sub.so =a predetermined amount of time, 
h=height of inner surface at dry point for the wash liquid, 
h=the changing height of the inner surface, and 
t.sub.s =the centrifuging time. 
Here the relationship of the filter-cake level at the dry point of the wash 
liquid h.sub.ET and at the dry point of the liquid phase of the charged-in 
suspension h.sub.Eto are formed. The speed reduction (dh/dt).sub.o at the 
dry point of the liquid phase and the speed dh/dt at the dry point of the 
wash liquid are calculated and set relative to each other. The thus 
obtained values are raised to the exponents b and c and then multiplied by 
the machine constant K.sup.a. Finally the thus obtained value is 
multiplied by the time value t.sub.so for a normal centrifuging step. The 
value t.sub.so can be calculated or determined empirically and does not 
change from fill to refill. 
It is also possible instead to use instead of the values h.sub.ETo and 
(dh/dt).sub.o values obtained from another centrifuging step or to put 
them together in a constant C so that 
EQU t.sub.s .about.C h.sub.Et.sup.b /(dh/dt).sup.c. 
It is also advantageous to optimize the constants a, b, c, K, and t.sub.so 
during operation of the centrifuge drum in succeeding uses. 
FIG. 4 shows the curves for two different products to be filtered, one in a 
solid line one in a dashed line. The dashed-line product has a larger 
particle size so that it filters faster. Once the maximum level is reached 
the fill valve is closed and the level drops more quickly for the 
suspension with the coarser solid fraction. For it, once the dry point 
W.sub.1 is reached the washing can start so that this procedure is 
finished at time R.sub.1 much earlier than the regenerating time R.sub.o 
of the suspension with a finer solids fraction. 
FIG. 5 schematically illustrates the control apparatus Con which receives 
from the sensor S the level and which itself keeps track of time to 
calculate the various velocities and curve slopes to control filling, 
washing, centrifuging, and stripping. In other words the values h and 
h.sub.ET are determined directly and compared with time by the controller 
Con. From the differential quotient dh/dt during filtering out of the 
liquid phase at dry point W and of the wash liquid at dry point ET the 
controller Con can calculate the centrifuging time in order to obtain a 
given residual moisture content at a point R. In determining the speed at 
which the liquid level drops in the rotating centrifuge drum it is 
possible instead of the differential quotient dh/dt to also used to use 
the average value of the linearized level decrease over time of the 
differential quotient .sub.D h/.sub.D t. Thus the controller Con can be 
analog or digital.