The invention relates to a multi-stage pusher centrifuge for the separation of a mixture into a solid cake and into a liquid phase, including an outer screen drum rotatable about an axis of rotation and at least one screen stage arranged in the outer screen drum, a mixture distributor arranged in the screen drum with a pusher base apparatus, with either the screen stage or the pusher base apparatus being arranged movably along the axis of rotation such that the solid cake is displaceable by means of the pusher base apparatus, and including an infeed device with which the mixture can be introduced via the mixture distributor into an empty space which forms when the solid cake is displaced by the pusher base apparatus.
Centrifuges are widespread and are used in the most varied areas in the most varied embodiments for the drying of moist substances or of moist substance mixtures. Discontinuously operating centrifuges such as scraper centrifuges are thus preferably used, for example, for the drying of very pure pharmaceutical products, whereas continuously operating pusher centrifuges are advantageously used in particular when continuously large volumes of a solid/liquid mixture should be separated. Depending on requirements, single-stage or multi-stage pusher centrifuges as well as double pusher centrifuges are used.
In the different types of the last named class of pusher centrifuges, a solid/liquid mixture, for example a suspension or a moist salt or salt mixture, is supplied via a mixture distributor through an inlet tube to a fast rotating drum which is designed as a filter screen such that the liquid phase is separated through the filter screen due to the acting centrifugal forces, whereas a solid cake is separated at the interior at the drum wall. A substantially disc-shaped pusher base with a synchronized co-rotation is arranged in the rotating drum, with—depending on the number of screen stages—either the pusher base or a screen stage oscillating at a specific amplitude in the axial direction in the drum such that some of the dried solid cake is pushed out at an end of the drum. On the movement of the pusher base in the opposite direction, a region of the drum adjoining the pusher base is released which can then be again loaded with a new mixture through the inlet tube and via the mixture distributor. Depending on the type used, throughput volumes in an order of magnitude of 100 tons per hour can be reached without problem with modern heavy-duty pusher centrifuges, with drum diameters of up to 1000 mm and more being quite normal and typical rotational frequencies of the drum being able to be achieved, depending on the drum diameter, of up to 2000 revolutions per minute and more. Due to the high centrifugal forces which occur, a larger drum diameter as a rule results in a smaller maximum rotational frequency of the drum. The operating parameters such as the rotational frequency of the drum, the volume of mixture supplied per time unit or also the drum diameter or the type of pusher centrifuge used can also depend on the actual material to be dried, the liquid content, etc.
The multi-stage pusher centrifuges known from the prior art are as a rule continuously operating filter centrifuges. The multi-stage filter centrifuge consists of an outer screen drum and at least one screen stage which is arranged in the outer screen drum and is likewise designed as a screen drum. A plurality of screen stages can be arranged concentrically inside one another such that two-stage, three-stage and multi-stage pusher centrifuges can be realized, with all screen stages being driven very fast synchronously about a joint axis of rotation. In the operating state, a solid/liquid mixture to be separated continuously enters through a fixed-standing inlet tube into a mixture distributor which is arranged in the innermost screen stage and which likewise rotates co-synchronously and is uniformly distributed on the innermost screen stage over its whole screen periphery. The largest part of the liquid is already centrifuged off here and a solid cake is formed.
In a two-stage pusher centrifuge, the innermost stage, which is also termed a first stage, carries out an oscillation movement in the direction of the axis of rotation in addition to the rotational movement about the axis of rotation. This oscillatory movement is, for example, generated hydraulically via a pusher piston with a reversing mechanism. The solid cake is thereby pushed from the first stage to the second stage in ring sections, corresponding to the stroke length of the oscillation, and ultimately exits the pusher centrifuge via a discharge opening. In practice, the solid cake is continuously washed in the screen drum while feeding washing liquid onto the solid cake.
A known two-stage pusher centrifuge which works in accordance with the aforementioned principle is described in detail, for example, in DT 25 42 916 A1. In two-stage and multi-stage pusher centrifuges, the first stage, i.e. the innermost screen stage, substantially serves for the pre-dewatering of the mixture as well as for the forming of a solid cake, whereas the outer screen drum mainly serves as a drying stage. Since a pre-dewatering is possible by means of the first screen stage, a much higher liquid absorption capacity is achieved with multi-stage pusher centrifuges so that mixtures with lower inlet concentrations, i.e. with a higher liquid content, can be processed.
For special areas of application, special versions of two-stage and multi-stage pusher centrifuges are known, in particular for highly abrasive centrifuge goods such as coal and raw phosphate, which require special abrasion protection measures such as abrasive-resistant screens. Special designs for intensive washing processes and for the carrying out of special washing methods such as counter-flow washing for nitro-cellulose are also known from the prior art. Gas-impermeable versions of multi-stage pusher centrifuges are also used for operation under an inert gas atmosphere.
Although multi-stage pusher centrifuges such as briefly outlined above have also been well known for special applications in the most varied variants for a long time, the known multi-stage pusher centrifuges nevertheless show different serious disadvantages. Even if lower inlet concentrations, i.e. mixtures with an increased liquid content, can be processed better with the known multi-stage pusher centrifuges than with customary single-stage pusher centrifuges, the inlet concentration of the mixture to be processed may not have any desired low degree. I.e. when the share of liquid in the mixture is too high, for example amounts to 50% or 70% or 80% or even more than 90% liquid phase, the mixture must be pre-condensed in more or less complex processes. With too high a liquid content, a uniform distribution of the mixture to be dried over the periphery of the screen drum is made increasingly difficult. This can result, on the one hand, in very damaging vibrations of the screen drum and thus to premature wear of bearings and the drive; in the worst case even to a safety problem in operation. On the other hand, a solid cake distributed unevenly over the periphery of the screen drum brings about problems in washing. Static condensers, arc screens or the very well known hydrocyclones are therefore available, for example, for the pre-dewatering. It is obvious that the use of such pre-dewatering systems is very complex and thus expensive both from a process and an apparatus point of view.
A further serious disadvantage in the processing of mixtures of a smaller inlet concentration consists of practically the whole volume of liquid supplied with the mixture having to be accelerated to the full peripheral speed before it is separated through the filter screen of the screen drum. The same applies to very small particles in the mixture which should likewise be separated from the solid cake through the screen. This is extremely unfavorable energetically and has a clearly negative influence on the operating behavior of the centrifuge.
But even in the processing of mixtures with a much higher solid concentration, the centrifuges known from the prior art have some huge disadvantages. For instance, the mixture introduced into the mixture distributor through the inlet tube is accelerated in a very short time up to the full peripheral speed of the drum on impacting the screen drum. This can result, among other things, in grain breakage, in particular with sensitive substances; that is, for example, solid grains which are distributed in a suspension supplied to the centrifuge burst into smaller pieces in an uncontrolled manner on the abrupt acceleration process, which can have negative influences on the quality of the solid cake produced when, for example, the particle size of the grains in the end product plays a role.