Note: Descriptions are shown in the official language in which they were submitted.
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DIGESTER
FIELD OF THE INVENTION
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This invention relates to a digester for treating
solids and especially for treating solids of a slurry feed
in order to leach out a wanted component of those solids. The
slurry feed may be an aqueous slurry, but is not necessarily
so as the digester may equally be used for acid leaching and
caustic leaching. Thus, examples of the uses to which the
digester may be put are the leaching of ores, such as copper
oxide, with sulphuric acid; the leaching of uranium ores
with a caustic solution; the leaching of a cons-tituent from
vegetable matter, such as the extraction of contained resin
from timber; and the leaching out of contained bitumen oils
from an aqueous slurry of oil sands. Where the liquid is
acid or caustic special ma-terials for the digester may be
necessitated.
BACKGROUND OF T~IE INVENTION
As described in copending patent application No:
386,017 filed 16th September 1981 in the name of RTR Riotinto
Til Holding S.A., one method of leaching out the bitumen
oil content of raw oil sands is to agita-te the oil sands
gently with relatively large quantities of hot water, the
lumps of oil sands being broken up by thermal action and
the bitumen oils released. It is further explained in that
application that the recovery oE the bitumen oils is facilit-
ated if, firstly, the liquid and solids are discharged
separately from the leaching process, and, secondly, clay
and other dross included in the oil sands are not disinteg~
rated during that process.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide
an improved digester for treating the solids of a slurry feed
in order to leach out a wanted component of those solids.
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I-t is a further object of the inven-tion to provide
an improved digester for the leaching of contained bitumen
oils from an aqueous slurry of oil sandsO
These and other objects of the invention will appear
from the following description.
SUMMARY OF THE INVENTION
In accordance with the present invention there is
provided a rotary digester for digesting solids comprising:
an outer shell;
means for entry of liquids and solids to be digested
at one end of the digester;
a tubular, axially extending outlet at the other end
of the shell for discharge of liquids;
a solids outlet at the same end as said liquids outlet,
surrounding, but separated from, the liquids outlet; and
a screw which surrounds the tubular liquids outlet to
urge solids to and through the solids outlet, which screw is
secured at its outer periphery to the shell.
It is to be understood that the reference to "solids
outlet" does not imply that dry solids only are discharged
through that outlet, as some liquid is perforce contained in
the solids. Similarly, the liquid discharged thorugh the
liquid outlet may contain minor quantities of solids, pre-
dominantly in the form of fines.
BRIEF DESCRIPI'ION_OF THE DRAWINGS
A preferred embodiment of the invention will now be
described, by way of example, with reference to the accompanying
drawings, wherein:
FIGURE 1 is a radial section through a digester in
accordance with the invention;
FIGURES 2 and 3 are sections on the line A-A of
Figure 1, the internal screw bars being omitted at the upper
half of Figure 2, and only those screw bars being shown in
Figure 3, and
FIGURE 4 is an axial section of the discharge end
of the digester.
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DESCRlP'rION OF THE ~REFE~RED E~BODI~FNTS.
Preferably the tubular liquids outlet is
surrounded by a screw to urge solids to and through the
solids outlet; advantageously the liquids outlet is
supported by the screw which is secured at its outer
periphery to the shell.
To prevent undue disintegration of large lumps
entered with the slurry, a cage may be mounted within,
but spaced from, the shell to allow passage
therethrough of solids below a given size, but to
prevent passage therethrough of oversize lumps, the
cage having a slurry inlet leading to the interior of
the cage. The cage preferably ter~inates short of the
tubular outlet and has openings which increase in size
at the outlet end to allow oversize lumps to pass
therethroua,h prior to solids discharge.
Within the space between the shell and -the
cage there may be means to urge solids in that space
axially towards the solids outlet.
The digester illustrated in the drawing is
constituted by a rotary drum having a shell 100,
mounted on rollers for rotation about a horizontal axis
and driven through a gearth gear (not shown).
Secured to the internal wall of the shell 100,
and extending over the larger part of the length of the
drum from one end, there are a number of
circumferentially spaced screw sections 101, in the
example illustrated, there are four such sections.
Each section consists of a series of axially spaced
flat bars 102, which are parallel and inclined to a
plane at right angles to the axis of the drum, as shown
in Figure 2. The bars 102 extend from the shell 100
~ ~ only a short distance, compared with the drum diameter.
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~etween consecutive pairs of screw sections 101 are
assemblies of stirrers 103. ~ach stirrer is a flat
har, which is secured to the internal wa]l of the shell
100, and which extends parallel to the s~ell axis; as
will be seen in Figure 1, the stirrers 103 are not
radially disposed, but each is inclined to the
respective radius.
The digester also has sets 104 of internal
screw bars 105, eight such sets being illustrated in
the drawing with one set a]igned with each of the screw
sections 101 and each assembly of stirrers 103. Each
screw bar 105 is rod like, al-though preferably it has a
square section, and is secured at one end to the
extremities of the flat bars 102. Further, as will be
apparent from Figures 2 and 3, the internal screw bars
105 in each set are inclined to the drum axis in
herring-bone style, while the bars of each set 104 are
offset from those of the adjacent set.
One end of the shell 100 is closed by an end
plate, having a central inlet for entry of feed
material. The other end, illustrated in Figure 4, is
open and has a central tubular liquid outlet 106, which
extends beyond the end of the shell 100 and over a
short axial length of the shell. The tubular liquid
outlet 106 is secured in place by a screw 107, the
outer periphery of which is welded or otherwise secured
to the interior wall of the drum 100. The annular
passage which surrounds the outlet 106 and which
contains the screw 107 constitutes a solids outlet.
The operation of the digester will be
explained in relation to the conditioning of mined oil
sands for the leaching out of the contained bitumen
oils, However, the digester may be used in other
processes where similar conditions apply.
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As is known, oil sands are subject to
disintegration and release of -the contained bitu~len
oils when treated with water at elevated temperature,
little mechanical agitation being needed. Further, it
is advantageous if the solids of the mined oil sands
can be separated from the liquid at an early stage in
the process. In order to simplify the extrac-tion of
the released oil sands, it is also desirable that the
liquid shall contain a minimum of fines.
With those ends in view, the oil sands are
treated by being mixed with hot water at an appropriate
temperature to form a slurry, which can contain lumps
of foreign matter such as clay, and which is introduced
into the digester drum through the inlet. The drum
itself is rotated in anti~clockwise direction as seen
in Figure 1 at a low rate of, for example, a few
revolutions per minute, the slow speed being chosen to
avoid break-up of -the clay lumps. The slurry enters
the central space within the internal screw bars 105;
those screw bars have a spacing such that the
relatively small masses of oil sands, and the sollds
from disintegrated oil sands, fall through the screw
bars into the annular space between them and the shell
100. On the other hand, large masses of clay are
prevented from entering that space and, initially, are
moved progressively alona the digester, by the screw
action of the bars 105.
The oil sands and solids that have fallen
through the internal screw bars 105 are gently agitated
by the stirrers 103 which, because of their
inclination, do not carry the solids far up the
digester during rotation of the shell. At the same
time, the solids are moved progressively axially along
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the shell 100 by the action of the sections 101 of flat
bars 102.
By the gentle agitation of the oil sands by
the stirrers 103, the particles of oil sands are broken
up, releasing the bitumen oils into the hot water,
while the remaining sand is retained between the
internal screw bars 105 and the shell 100. Towards the
discharge end of the shell 100, the spacing of the
internal screw bars 105 is increased, thus allowing the
clay masses to fall through them to join the now oil-
free sands. The inclina-tion and spacing of the flat
bars 102 are such that the solids move axially along
the shell at a speed which is substantially less than
the throughput of liquids.
At the discharge end of the drum, the liquids
- the hot water and the bitumen oils from the oil sands
- are discharged through the central tubular outlet 106
and are treated for the extraction of the oil sands, as
described in the above-mentioned patent application.
At the same time, the solids, which are progressed
along the bottom of the drum by the screw action of the
flat bars 104, are discharged by the screw 107 through
the solids outlet surrounding the liquids outlet 106.
The solids, which contain some water and a little oil,
are subsequently treated for recovery of the oil and
separation of the water, before being returned as
infilling.
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