Liquid purification or separation – Processes – Separating
Reexamination Certificate
1999-06-02
2001-06-05
Reifsnyder, David A. (Department: 1723)
Liquid purification or separation
Processes
Separating
C210S787000, C210S360100, C210S374000, C210S380100, C494S037000, C494S052000, C494S053000, C494S054000, C494S056000
Reexamination Certificate
active
06241901
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a centrifuge and to an associated method of operating a centrifuge. The apparatus and method of the invention are particularly, but not exclusively, applicable in cantilever screen-scroll type centrifuges.
Conical screen-scroll centrifuges have been used to dewater thickened slurries from nominally 40-60% feed solids to nominally 80-95+% solids (or 20-5% cake moisture). As illustrated in
Fig. 1A
, such centrifuges comprise a scroll conveyor
10
surrounded by a screen basket
12
and disposed therewith in a housing
14
. Scroll conveyor
10
and screen basket
12
are cantilevered from a support
16
at one end. At that same end, conveyor
10
and screen basket
12
are operatively connected to a single input, dual output planetary gear box or a cyclo gear box
18
which is driven by a motor
20
. A feed pipe
22
extends into an open, free end of scroll conveyor
10
for delivering a thickened feed slurry thereto. The feed slurry exits an opening (not shown) in a hub
24
of conveyor
10
and is deposited onto screen basket
12
. Solids
26
in the slurry are conveyed along an inner surface of screen basket
12
to a conical discharge
28
by a helical blade
30
of conveyor
12
, while filtrate is discharged at
32
through screen basket
12
.
A simple cross-sectional schematic of the screen-scroll centrifuge of
FIG. 1A
is shown in FIG.
1
B. Feed slurry introduced via feed pipe
22
into a feed cone
34
of conveyor
10
is accelerated in the feed cone (arrows
36
) so that when the slurry is laid onto a small diameter end
38
of screen basket
12
, the slurry has acquired the proper G-force to effect filtration of the bulk liquid followed by dewatering (arrows
32
) so that the remaining liquid trapped in the cake pores can be further released with time. The dewatering process is facilitated by continuously thinner cake and an increasing higher centrifugal force as the cake moves toward discharge at a larger screen diameter
42
. Washing can be applied to remove the impurities in food, chemical, and mineral applications, wash liquid being introduced at small diameter
38
of conical screen basket
12
shortly after the feed zone. The washed cake is ultimately dewatered at the larger screen diameter
42
. The screen drain filtrate (arrows
32
) and the cake (arrow
44
) are collected respectively in separate hoppers (not shown) for downstream processing.
One key benefit of the cantilever screen scroll design as illustrated in
FIGS. 1A and 1B
, is that both scroll conveyor
10
and screen basket
12
are opened at the front end of the machine. This allows the operator easy access to the rotating assembly for regular maintenance such as replacement of worn components (e.g. screen, worn and broken tiles, scroll, nuts and bolts), and removal of foreign objects trapped in the process streams, as well as regular visual inspection of the process during operation to assure satisfactory operation. Because the screen scroll centrifuge is a cantilever design, another advantage is that only a set of supporting bearings located at one end of the machine is required instead of two bearings associated with a horizontal end-to-end support. This minimizes significantly the overall cost of the machine. However, there is a disadvantage in that the overhung moment from the pivot or support may limit the cantilever mass as well as the distance of cantilever mass from the pivoted bearing or support. This may also result in a rotational speed limitation owing to natural frequency considerations. Another limitation of the screen-scroll-type centrifuge is that the feed has to be pre-thickened to nominally 40-60% before introduction to the screen to remove a majority of the bulk liquid. This thickening can be achieved, for example, with hydrocyclones, thickening tanks or thickening screens upstream of the dewatering screen scroll.
In a different approach, both thickening and dewatering are combined in a single unit using a screen bowl centrifuge as shown in
FIG. 2. A
solid-bowl configuration comprises a cylindrical bowl
46
followed by a conical beach
48
used for separation and thickening of the separated solids to form a cake. A cylindrical screen
50
downstream of the conical beach is used to further dewater the cake to lower the moisture content thereof. Consequently, dilute feed with solids content by weight of 5-50% can be used. This is advantageous over the screen scroll where only thickened feed of nominally 40+% is permissible.
The prior art centrifuge of
FIG. 2
also includes a worm-type conveyor
52
for scrolling cakes solids along inner surfaces of bowl
46
, beach
48
, and screen
50
. Effluents are discharged from a clarifier pool
54
into a centrate discharge chamber or hopper
56
of a centrifuge casing
58
. Filtrate is discharged through screen
50
into a filtrate drainage chamber or hopper
60
of casing
58
, while cake
62
is discharged into a solids discharge chamber or hopper
64
. A feed slurry is fed into a hub
66
of conveyor
52
via a feed pipe
68
. Conveyor
52
and bowl
46
are rotatably supported at opposite ends on bearings
70
and
72
and are differentially rotated via a gear unit
74
.
In another variation of the screen-bowl-type centrifuge, shown is
FIG. 3
, a cylindrical screen section
76
is provided at a larger diameter than the diameters of a cylindrical solid bowl section
78
and a bowl section
80
. A first helical conveyor blade
82
conveys cake solids along inner surfaces of bowl section
78
and bowl section
80
, while a second helical conveyor blade
84
conveys cake solids along an inner surface of screen section
76
. Conveyor blades
82
and
84
are rigid with a conveyor hub
86
and accordingly rotate at the same angular velocity which is slightly different from an angular velocity of screen section
76
, bowl section
78
and bowl section
80
.
An advantage of the design of
FIG. 3
is that cake dewatering on screen section
76
is carried out at a higher G-force. A disadvantage is that as the feed as laid abruptly onto screen
76
, the feed is underaccelerated, i.e., the tangential speed of the feed is much less than that of screen
76
at a solid-body rotation. This difference in tangential speed results in slippage of the feed on the screen surface as the feed is being accelerated by the screen surface, thereby causing high wear on screen
76
especially for abrasive feed materials. Furthermore, it can be shown that the undesirable radial velocity of the feed stream increases at the expense of a lower tangential speed (conservation of angular momentum). This in turn results in an increased solids penetration through screen
76
, with a lower solids recovery or capture. The feed particle size can be further reduced through slippage of feed on the screen with the consequence of particle attrition which results in more loss of these fine solids through the screen. In all cases of this variation of the screen-bowl-type centrifuge, the screen bowl is horizontally arranged and supported by two bearings
88
(only one shown) at the two ends. The cost of this design is somewhat greater than the cantilever screen scroll design (
FIGS. 1A and 1B
) and the operator cannot access the rotating assembly as readily as in a cantilever screen scroll design.
An improvement in that direction is a cantilever screen bowl design as shown in FIG.
4
. The unit includes a cylindrical bowl
90
and a conveyor
92
both rotatably cantilevered from a support located at the large diameter side of the machine. Because of this arrangement, in order to reduce the overhung bending moment, the length of the solid-bowl section
90
as well as the length of a cylindrical screen section
94
must be trimmed. The rotational speed of the machine may also limited owing to natural frequency considerations. These factors render the overhung shorter screen bowl design less effective with major disadvantageous results of lower throughput, wetter cake and dirtier effluent as compared to a regular scr
Baker Hughes Incorporated
Coleman Henry D.
Reifsnyder David A.
Sudol R. Neil
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