GB2026333A - Sugar centrifuging - Google Patents

Abstract

In a sugar centrifuge 3 which separates crystals from liquid by causing the crystals to build up on a screen, 11 through which the liquid passes compacted sugar crystals are removed from the screen after centrifuging by using a very high braking force to stop rotation of the centrifuge very suddenly. The rotation of the screen ceases but the crystals continue to rotate due to their rotational inertia and so detach themselves from the screen. The centrifuge may include one or more apertures 17 in the base 16 of the drum through which the sugar crystals leave the centrifuge. The centrifuge can be rotated at over 1200 r.p.m. and brake means 25 are applied to bring the centrifuge to rest. Plate 13 on driving shaft 7 deflects incoming liquid from pipe 2 radially outwards onto rotating screen 11. <IMAGE>

Description

SPECIFICATION Sugar centrifuging This invention relates to sugar centrifuging.

Centrifuges used in the refinement of sugar are required to separate the particulate sugar crystals from the syrup fraction of the massecuite, magma, or mother liquor. In general, the centrifuge causes a mixture to be separated to pass through a perforated surface, such as a screen annularly mounted within a centrifuge drum. The screen has apertures sufficiently small to retain sugar crystals included in the mixture, but the syrup passes through the perforations for exit through an exit port. The sugar crystals accumulate on the screen and are compacted by centrifugal force. After the mixture has been separated, the sugar crystals remain against the screen and must be removed by some means. It has heretofore been common to scrape the inside wall of the screen to remove or "discharge" the sugar crystals.

Certain disadvantages attend this form of discharging the crystals. First, time is required to remove the sugar crystals from the centrifuge before the next charge may be admitted.

Also, the crystals are occasionally damaged due to the scraping action. Finally, the discharging apparatus gradually wears out the screen within the centrifuge chamber.

According to a first feature of the present invention there is provided a method of operating a sugar centifuge which comprises introducing into a perforate walled drum of the centrifuge a mixture of sugar crystals and liquid, rotating the drum to allow sugar crystals to collect on the perforate wall and the liquid to pass through the perforations, and bringing the drum to a halt sufficiently rapidly to cause sugar crystals on the wall to detach from the wall.

More specifically, the invention provides a method of separating sugar crystals from fluid matter containing sugar crystals, which comprises: rotating the fluid matter at a speed greater than 1 200 rpm to subject it to a centrifugal force; restraining sugar crystals, urged by the centrifugal force, from movement beyond a predetermined point defined by a screen means fixedly mounted on the inside surface of a rotatably mounted drum; simultaneously allowing undesired portions of the fluid matter to pass away from the crystals and through the screen means, and thereafter applying a braking force to bring the drum to substantial rest in less than approximately one second, causing the crystals to disengage from the screen means and continue rotation because of their inertia, and collecting the crystals.

The invention also provides a sugar centrifuge of the type having a rotatable surface for retaining sugar crystals, the surface being rotatable at speeds greater than 1 200 r.p.m.

the centrifuge comprising: brake means associated with the surface capable of decelerating the surface from over 1000 r.p.m. to substantial rest in less than approximately one second whereby to cause compacted sugar crystals to disengage from the surface.

By providing a brake associated with the centrifuge drum able to decelerate the spinning centrifuge drum to a sudden stop, the provision of other discharge equipment may be dispensed with. The rapid deceleration of the centrifuge drum and screen causes compared sugar crystals to break loose from the screen. The crystalline mass continues rotating due to the rotational inertia imparted during centrifugation, but eventually the mass falls or slides down the screen under the force of gravity and it may be allowed to exit from the centrifuge via one or more exit apertures at the bottom of the centrifuge drum.

The invention is described further, by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic side view of the internal parts of a centrifuge according to the present invention, and Figure 2 is a sectional view along the line 2-2 of Fig. 1.

Referring to the drawings, the internal parts of a centrifuge 3 include a cylindrical drum 5 mounted on a shaft driven by a variable speed motor 9. A finely perforated screen 11 is securely mounted coaxially within the centrifuge drum 5.

Positioned centrally within the drum is a device to prevent spillage of massecuite during charging of the drum. In the embodiment shown, this takes the form of a circular plate 13 mounted on shaft 7.

In operation, the centrifuge drum 5 is set in motion by the motor 9 driving shaft 7. The centrifuge basket should be rotated at a speed of 1,200 to 1,400 r.p.m. for high grade massecuite or mother liquor, and for lower grades, preferably rotated at 1,800 to 2,200 r.p.m.

After the centrifuge has reached speed, mother liquor or massecuite 1 is admitted through an inlet 2 in the top of the centrifuge 3. Massecuite falling on to plate 1 3 is directed radially outward until it reaches the perforated screen 11. The perforations in the screen are small enough to retain the sugar crystals, while the syrup passes through the perforated screen 11 to the centrifuge drum 5. Drum 5 is also perforated to allow the syrup to pass through the drum wall and collect in fluid jacket 1 8 whence it may be removed via a syrup outlet 1 9 in the bottom of the jacket 18.

After separation, which for first and second grades of massecuite takes approximately two minutes, the sugar crystals accumulated against the screen 11 will have formed a compacted mass 12. The centrifuge is then decelerated to approximately 100 to 200 r.p.m., after which a substantial braking force is applied to the drum. Any conventional braking system may be used; in the embodiment shown brake shoes 25 engage a brake disc 23 mounted coaxially with and at the top of drum 5.

The rapid deceleration arising from the substantial braking force stops the spinning centrifuge drum 5 and screen 11, but the compacted sugar crystals 1 2 break free of the screen 11 and continue in their rotation due to their rotational inertia. Gravity causes crystals 1 2 to descend the wall of centrifuge drum 5, ultimately reaching the base 1 6 of the drum. The base 1 6 includes a plurality of exit apertures 1 7 through which the sugar crystals may pass. After leaving the centrifuge drum 5, the sugar crystals exit the centrifuge through a sugar crystal outlet 21 for collection.

Claims (9)

1. A method of separating sugar crystals from fluid matter containing sugar crystals, which comprises: rotating the fluid matter at a speed greater than 1 200 rpm to subject it to a centrifugal force; restraining sugar crystrals, urged by the centrifugal force, from movement beyond a predetermined point defined by a screen means fixedly mounted on the inside surface of a rotatably mounted drum; allowing undesired portions of the fluid matter to pass away from the crystals and through the screen means, and thereafter applying a braking force to bring the drum to substantial rest in less than approximately one second, causing the crystals to disengage from the screen means and continue rotation because of their inertia, and collecting the crystals.

2. A method according to claim 1 wherein the braking force is sufficient to reduce the rotational speed from 200 r.p.m. to zero in less than one second.

3. A method according to claim 1 or 2 wherein the fluid matter is rotated at-a speed between 1200 and 1400 r.p.m.

4. A method according to claim 1 or 2 wherein the fluid matter is rotated at a speed between 1800 and 2200 r.p.m.

5. A method according to any one of claims 1 to 4 wherein the fluid matter is first introduced into the interior of the rotatable drum only after the drum has reached the desired rotational speed and wherein the drum contains means for deflecting the fluid matter so introduced outwardly toward the screen means.

6. A method according to any one of claims 1 to 5 wherein the rotatable drum includes a base portion including at ieast one aperture and the sugar crystals are collected by passing the crystals through the aperture.

7. A method of operating a sugar centrifuge which comprises introducing into a perforate walled drum of the centrifuge a mixture of sugar crystals and liquid, rotating the drum to allow sugar crystals to collect on the perforate wall and the liquid to pass through the perforations, and bringing the drum to a halt sufficiently rapidly to cause sugar crystals on the wall to detach from the wall.

8. A method of operating a sugar centrifuge substantially as hereinbefore described with reference to the accompanying drawings.

9. A sugar centrifuge of the type having a rotatable surface for retaining sugar crystals, the surface being rotatable at speeds greater than 1 200 r.p.m., the centrifuge comprising: brake means associated with the surface capable of decelerating the surface from over 1000 r.p.m. to substantial rest in less than approximately one second whereby to cause compacted sugar crystals to disengage from the surface.