A diffusion apparatus for extraction of sugar from sugar cane has a vertically-diposed annular extracting compartment including a revolving screen deck contained within a fixed inner shell and a fixed outer shell. The revolving screen is supported on rollers and driven by any suitable elements. The comminuted sugar cane is fed into the compartment and extracting liquid is moved countercurrent and exits through an outlet.

FIELD OF THE INVENTION
 Although this specification makes particular reference to the extraction of
 sucrose from sugar cane, it must be understood that the invention also
 covers the extraction of any suitable soluble substances from any type of
 sub-divided solids containing such substances.
 The extraction of sucrose from sugar cane generally comprises two major
 stages: firstly, a particle size reduction stage, during which the cane is
 prepared to a certain fineness; and secondly, the crushing and/or the
 diffusion, or lixiviating, stage. In this latter stage, the prepared cane
 or bagasse is conveyed and by percolation and/or countercurrent flowing of
 a suitable water mixture, the residual sucrose is dissolved and extracted.
 The present invention is directed to the second stage.
 BACKGROUND OF THE INVENTION
 Various apparatus and methods are known to effect the diffusion stages.
 It is an object of the invention to provide an apparatus for solvent
 extraction by means of which the movement of the prepared cane or bagasse
 is relatively simplified.
 SUMMARY OF THE INVENTION
 According to the invention there is provided a diffusion apparatus
 comprising a vertically-disposed annular extracting compartment, a means
 of feeding the compartment with the material being processed, a means to
 move the material around and within this compartment, and a means of
 removing the material from the compartment.
 The extraction stage may comprise a revolving screen deck contained within
 a fixed inner column, or shell, and a fixed outer column, or shell. The
 revolving screen is supported on rollers and is caused to revolve by
 suitable drives, such as hydraulic motors or rams, or by electrical
 geardrives. More than one drive mechanism may be employed.
 The material being processed rests on the screen deck and is transported
 from the feed inlet of the annular compartment to the discharge outlet of
 the compartment as the screen deck revolves.
 The material is fed into the annular compartment by means of a distributing
 conveyor and is removed at the discharge outlet by means of discharge
 screws, or other means of discharging the material, such as discharge
 chain conveyors, or a suitably shaped discharge plate. The discharge
 mechanism feeds the material into a discharge chute in the outer fixed
 column, or shell.
 In a preferred form of the invention the outer and inner shells of the
 apparatus are fixed. Within the two shells is a rotating screen deck
 supported over several fixed trays. During the passage of the material to
 be processed from the inlet to the discharge stages the material is
 subjected to a countercurrent spraying of a solvent, the solute being
 collected in the trays beneath the screen deck and pumped in a
 countercurrent fashion from the discharge stage where the solute is least
 concentrated to the inlet stage where the solute is most highly
 concentrated. The solute is then sent for further processing and the
 material is discharged for final solvent removal. In the case where the
 material prepared is cane the solute can be water or cane sugar juice of
 varying sugar concentration.

DETAILED DESCRIPTION OF THE INVENTION
 Referring to FIG. 1, the diffuser consists of an outer fixed cylindrical
 shell, 1, an inner fixed cylindrical shell, 2. and a rotating screen deck,
 3, supported over several fixed juice trays, 4, by means of rollers, 5.
 The rotating screen deck is rotated by means of one or more electrical or
 hydraulic geared motors, 6, or by stepless hydraulic cylinders. (Not
 shown)
 The fixed outer column or shell, 1, and the fixed inner column, 2, are
 joined together at the base by the fixed juice collection trays, 4, and
 support beams, 7, and at the top by a fixed roof structure, 8.
 Juice collected in the trays, 4, is pumped countercurrent to the flow of
 the material by pumps, 9, and sprayed on to the upstream material through
 sprays, 10. The spray area can be adjusted by means of spray directing
 flaps, 11. Vertical bed disturbance screws, 12, are placed in areas of the
 bed that require reforming to avoid bed clogging, or blinding.
 Referring to FIG. 2, the prepared material is fed onto the slowly rotating
 screen by means of a feed conveyor, 13, and the material forms a bed, 14.
 The bed is saturated with solute by means of sprays, 10. which are
 situated over the bed at fixed positions to provide a counter-current flow
 of solute relative to the flow of the material. The slowly rotating deck
 carries the bed of material from the inlet at conveyor, 13. to the
 material discharge opening, 18. The material is removed from the rotating
 deck by horizontal discharge screws, 16, and fed to a discharge conveyor,
 19.
 Referring to FIG. 3, this a sectional plan D--D of the diffuser showing the
 fixed outer shell, 1, the fixed inner shell, 2; the rotating screen deck,
 3; juice pumps, 9; bed disturbance screws, 12; material feed conveyor, 13;
 dewatering roll, 15; discharge screws, 16; fixed inner wall, 17; discharge
 opening, 18; and a material discharge conveyor, 19. Material, prepared
 cane, enters the diffuser from openings in the bottom of the material feed
 conveyor, 13, and falls to the rotating deck, 3, to form a bed of
 material. The rotating deck is driven by two or more electrical or
 hydraulic drives, 6. The material, prepared cane, Is transported from the
 feed end of the diffuser In one revolution to the discharge opening, 18,
 during which it is sprayed by recirculating juice from pumps, 9. For
 clarity the sprays are not shown in FIG. 3. The bed is disturbed by
 disturbance screws, 12, at predetermined positions. A dewatering roll, 15,
 is placed before the discharge opening, 18, to prevent flooded material
 from leaving the diffuser in an uncontrolled manner. A fixed inner wall,
 17, separates the feed inlet of the diffuser from the discharge end.
 FIG. 4 represents the process flow diagram of a cane diffuser. Prepared
 cane is fed into the feed area of the diffuser and saturated with scalding
 juice to raise the temperature and start the diffusion process. The bed
 formed on the moving screen deck is sprayed with juice, which percolates
 down through the screen deck into fixed trays underneath from where it is
 pumped in a countercurrent manner back on to the bed. Press juice, or hot
 water or both, is applied near the discharge area of the diffuser and
 surplus juice is discharged for further processing near the feed end of
 the diffuser where the juice has the highest concentration of sucrose due
 to the countercurrent flow of the process. The points at which juice is
 extracted from a juice tray compartment and pumped counter-current to the
 flow of bagasse is illustrative only and can be varied according to the
 permeability of the bagasse mat and amount of stage recirculation
 required,
 FIG. 5 shows a vertical cross-section through a typical stage, indicating
 the means whereby juice is pumped from a juice tray under the fixed screen
 deck and sprayed onto the moving bagasse mat in a counter-flow fashion.
 The sprayed juice percolates down through the bagasse mat, through the
 screen and into the juice trays to be pumped again in a counter-current
 manner. The porosity of bagasse mat affects the time taken for the juice
 to percolate down through the moving bagasse mat and thus the point of
 exit of the juice through the screen deck into the trays. It may be
 preferable to have some means of adjusting the point of application of the
 juice sprays, 10, so that the desired exit point of the juice from the
 bagasse mat is controlled, and this is obtained by varying the position of
 the spray directing flaps, 11.
 The advantages of the invention are seen to be as follows:
 1. Construction is mainly simple platework and relatively inexpensive.
 2. There are no chains, or expensive chain driving systems necessary to
 transport the cane within the diffuser.
 3. The rotating screen deck rests on rollers and does not require heavy
 torques to be applied to headshafts, tailshafts, or chains, as in
 conventional longitudinal diffusers in present use, to cause the deck to
 move.
 4. The rotational torque is applied at the outside diameter of the deck at
 a point of maximum leverage thereby reducing the force required. The
 rotation may be applied by synchronised electric or hydraulic motors, or
 by synchronised hydraulic rams to obtain a steplessly variable rotational
 speed inexpensively.
 5. No heavy chains or scrapers rest on the screens thereby reducing the
 torque required for rotation and reducing the wear to stationary members
 caused by chain transporting systems.
 6. It is suitable for outdoor operation and is totally enclosed.
 7. There is only one major moving part in the diffuser, apart from pumps,
 and that is the slowly rotating screen deck.