The present invention concerns a procedure for clarifying liquid suspensions, particularly those produced in processes of the wood- processing industry, wherein fibrous solid matter is removed from a liquid.

Noticeable quantities of liquid suspensions are produced in various processes in the pulp and paper industry, in which suspensions the content of solid matter is very low. Liquids of this kind are for instance the filtrate from the washing and bleaching process of a sulphate pulp mill, the ^" backwater (zero water) of a paper mill, the effluent water produced in manufacturing mechanical pulp and the barking plant effluent. At present, close enough removal of the low contents of solid matter present in these large liquid quantities is a proDlem. Economical removal would render possible a high degree of recovery and thereby ^" better utilization of the raw ma¬ terial, as well as processes which are more closed, whereby the environmental detriments caused by the mills would ^" be reduced.

The clarifiers in present use operate according to the sedimenta¬ tion principle. The liquid containing solid matter is conducted into a large,, circular basin. The feed flow of the liquid is small compared with the capacity of the ^" basin., whereby the delay of the liquid in the basin is prolonged and the solids in the liquid have time to settle on the bottom of the basin.. From the bottom, the solids are collected with the aid of scrapers. The clarified liquid is conducted over an overflow sill on the side of the ^" basin to further treatment or discharged through a drain into a water course.

With clarifiers in present use in the pulp and paper industry, the following typical contents of solids in the clarified water are obtained, depending on the subject treated: with the fibre recovery

clarifier of a pulp mill, 10 to 60 mg per litre, with the fibre recovery clarifier of a mechanical pulp mill, 50 to 200 mg per litre, with the barking plant effluent clarifier, 500 to 1500 mg per litre.

The object of the present invention is to provide a clarifying procedure by which the separation of lesser contents of solid matter than ^" before from large quantities of liquid is economically possible. The invention is characterized in that the clarifying takes place in a fluidized bed reactor in a liquid flow rising upward from below, and wherein a layer of fibre agglomerate is established on top of the finely divided, floating particles, which agglomerate binds to itself fibrous material from the liquid, flowing through the layer, and that fibre agglomerate is removed from the reactor as required, as the layer increases in bulk.

It is essential with a view to the establishment of the fibre ag¬ glomerate layer that the flow of the liquid suspension is levelled out in. the region, of the fluidized layer over the entire cross section of the reactor by the aid of suitable controls. In a com¬ pletely placid flow like this, the fibrous solid matter present in the liquid begins to form a reticular structure, of which in the further course of the clarification process is formed a continu¬ ously growing fibre agglomerate layer.

As the fibre agglomerate layer grows, agglomerate may be removed periodically or continuously from the reactor by the aid of one or more drain conduits.

An advantageous embodiment of the invention is characterized in. that the liquid to be clarified is circulated from on top of the fibre agglomerate layer to the bottom of the reactor. By the aid of this circulation,the rate of flow in the reactor may be adjusted to be proper, independent of the amount of feed flow. For instance, when the clarification process is started, the rate of flow should be kept low, and when the growth of the agglomerate layer has come under way, the rate of flow may be increased by means of the

circulation.

The floating particles to be used in the reactor are preferably

3 particles with specific gravity between 1.1 and 2.0 kg/dm and with a diameter between 0.2 and 3«0 mm. The particles may for instance consist of anion resin.

The invention is described in the following in greater detail to begin with by referring to the attached drawing, wherein Figs 1 and 2 present two different installations for applying the procedure of the invention, and thereafter by describing the procedure by the aid of an exemplary case.

The principal component of the apparatus depicted in Fig. 1 of the drawing is a fluidized bed reactor 1 composed of a tank with cir¬ cular cross section. In the reactor 1 , liquid suspension containing fibrous solid matter is clarified, the suspension being supplied into the reactor by a feed line 3 provided with a pump 2. The feed line 3 terminates on the bottom of the reactor 1 , and from the top end of the reactor starts the removal pipe 4 for the clarified liquid. In addition, the apparatus comprises a circulation line 6 provided with a pump 5 , this line joining the feed line 3 at a point between the pump 2 and the reactor bottom.

In the lower part of the fluidized bed reactor 1 , there is a layer of finely divided particles 7 which are maintained in floating state by the flow supplied into the reactor f through the line 3- In the conically widening part of the rea^or 1 have furthermore been placed conical baffles 8, which equalize the flow over the whole cross section of the ractor. Above the floating particles 7 in the course of the clarification a continuously growing fibre agglomerate layer 9 is formed, which is produced from the fibrous solid matter in the liquid suspension to be clarified. For removal of the fibre agglomerate, the reactor 1 has been provided with draining lines 10, starting at the agglomerate layer 9« The space between the fibre agglomerate layer 9 and the liquid drain line 4 becomes filled with a layer of water 11 clarified in the reactor.

In Fig. 2 is shown an apparatus wherein the fibre agglomerate draining lines connected to the sides of the fluidized bed reactor 1 have been replaced by a single vertical removal line 10 starting from the centre of the agglomerate layer 9- In all other respects, this apparatus is equivalent to that of Fig. 1.

Example

With an apparatus according to Fig. 1 of the drawing, the filtrate from a drum bleaching line was clarified. In the fluidized bed reactor, with diameter 6.1 m, anion. resin, particles of diameter 0.2-2.0 mm were.used as floating particles. The quantity of anion resin particles was approximately 1% of the reactor volume. During the clarification a layer of fibre agglomerate was formed in the reactor, its upper limit-remaining level with the end of the cir¬ culation pipe, as a result of the continous circulation applied. . The quantity of filtering liquid fed into the apparatus, respec¬ tively that of clarified water extracted from the apparatus, was 60 litres per second and the circulation flow rate was 100 to 200 litres per second. The quantity of solid matter in the filtrate fed into the apparatus was 30 to 50 mg per litre, in. the fibre agglom¬ erate layer, respectively in the circulation flow, 300 to 400 mg per litre on the average, and in. the clarified water removed from the apparatus, 5 to 1 mg per litre. During the experiment, fibre agglomerate was removed from the reactor at intervals of 3-7 days so that the boundary of the agglomerate layer desccended in con¬ nection with each removal approximately to the halfway point of the reactor volume.

Comparison of the solid matter content of the water removed from the reactor with those achievable with the aid of clarifiers of prior art reveals the substantial improvement of the clarifying results obtained by the procedure of the invention..

It is obvious to a person skilled in the art that different embod¬ iments of the invention are not confined to the examples presented

and may instead vary within the scope of the claims stated below. For instance, as floating particles may be used, instead of anionic particles, sand, active carbon or any other suitable material having a specific gravity which is higher than that of the liquid to be clarified.