FI78524C - Process for clearing liquid suspensions that arise in particular in the processes of the wood processing industry - Google Patents

Description

78524

METHOD OF CLARIFYING LIQUID SUSPENSIONS, IN PARTICULAR IN PROCESSES IN THE WOOD-PROCESSING INDUSTRY -

The present invention relates in particular to a process for clarifying liquid suspensions in processes in the wood processing industry, which process removes fibrous solids from a liquid by introducing the liquid into a reactor, binding the solids to a fibrous agglomerate layer formed in the reactor and growing the layer.

The various processes in the pulp and paper industry produce considerable amounts of liquid suspensions with a very low solids content. Such liquids are, for example, the filtrate from the washing and bleaching process of the pulp mill, the circulating water (O-water) of the paper mill, the effluent from the production of mechanical pulp and the peel effluent. Removing these small solids from these large volumes of liquids with sufficient accuracy is currently a problem. Economic disposal would allow for a high recovery rate and thus better utilization of the raw material, as well as more closed processes, thus reducing the environmental damage caused by the mills.

The clarifiers currently in use operate on the 1-step principle. The solid-containing liquid is led to a large circular basin. The liquid supply flow is small compared to the volume of the basin, whereby the delay of the liquid in the basin is long and the solids contained in the liquid have time to settle to the bottom of the basin. At the bottom, the solid is collected with scrapers. The clarified liquid is discharged through an overflow threshold at the edge of the basin for further treatment or discharged through a sewer into the water body.

Depending on the site, the clarifiers currently used by the pulp and paper industry achieve the following typical clarified water solids contents: pulp mill fiber recovery clarifier 10-60 mg / l, mechanical pulp fiber recovery clarifier 50-200 mg / l, shell water clarifier 500-1500 mg / l.

It is an object of the present invention to provide a clarification method which makes it economically possible to separate even lower solids contents from large amounts of liquid. The invention is characterized in that the clarification takes place in a bottom-up liquid stream in a Lei fluidized bed reactor, where a fibrous agglomerate layer is formed above the fluidized bed of finely divided particles so as to bind the fibrous material from the liquid flowing through the bed.

It is essential for the formation of the fibrous agglomerate layer that the flow of the suspension is smoothed by means of suitable guides in the region of the fluidized bed over the entire cross-section of the reactor. In such a completely calmed flow, the fibrous solid in the liquid begins to form a reticulated structure, from which a continuously growing fibrous agglomerate layer is formed as the clarification process continues.

As the fibrous agglomerate layer grows, the agglomerate is removed from the reactor from time to time or continuously by means of one or more discharge lines.

A preferred embodiment of the invention is characterized in that the liquid to be clarified is circulated from the top of the fibrous agglomerate layer to the bottom of the reactor. With the help of recirculation, the flow rate in the reactor can be adjusted appropriately regardless of the amount of feed flow. For example, when starting the clarification process, the flow rate should be kept low, and once the agglomerate layer has grown, the flow rate can be increased by utilizing recycling.

The floating particles used in the reactor are preferably 3 particles with a specific gravity of 1.1-2.0 kg / dm and a diameter of 0.2-3.0 mm. The particles may be e.g. anionic resin.

The invention will now be described in more detail with reference, first, to the accompanying drawing, in which Figures 1 and 2 show two different apparatus for applying the method according to the invention, and then describing the method by means of an example case.

78524

The main part of the apparatus shown in Fig. 1 of the drawing is a fluidized bed reactor 1 formed by a tank with a circular cross-section. The supply line 3 terminates at the bottom of the reactor 1, and a clarified liquid discharge line 4 begins at the upper end of the reactor.

In the lower part of the fluidized bed reactor 1 there is a layer of fine particles 7, which are kept in a fluidized state by the flow fed to the reactor along the line 3. In addition, conical plates Θ acting as flow guides are placed in the conically expanding part of the reactor 1, which smooth the flow over the entire cross-sectional surface of the reactor. Above the floating particles 7, a continuously growing fibrous agglomerate layer 9 is formed during clarification, which is formed from the fibrous solid contained in the liquid suspension to be clarified. To remove the fibrous agglomerate, the reactor 1 is provided with effluents 10 starting at the agglomerate layer 9. The space between the fibrous agglomerate layer 9 and the liquid effluent 4 is filled in the reactor with a layer of clarified water 11.

Fig. 2 shows an apparatus in which the fiber agglomerate discharge lines connected to the sides of the fluidized bed reactor 1 have been replaced by a single vertical discharge line 10 which d? Q, the pilomerate ti in the middle of the bed 9. In all other respects, this apparatus corresponds to that shown in Figure 1.

Example

The filtrate of the drum bleach line was clarified with the apparatus of Figure 1 of the drawing. In the fluidized bed reactor with a diameter of R, 1 m, anionic resin particles with a diameter of 0.2-2.0 mm were used as fluidized particles. The amount of anion-resin particles was about 7 l in the reactor volume. During S ei 1 boiling, a fibrous agglomerate layer formed in the reactor, the upper limit of which remained at the end of the recycle line due to continuous recycling. The amount of filtrate fed to the equipment and the clarified water removed from the equipment, respectively, was 60 1 / s, and the recirculation flow was 100-200 1 / s. The amount of solids in the filtrate fed to the equipment was 30-50 mg / l, in the fiber tuagglomerate layer and in the recycle flow, respectively, on average 300-400 mg / l and in the clarified water removed from the equipment 5-15 mg / l. During the experiment, the fibrous agglomerate was removed from the reactor every 3-7 days so that the boundary of the agglomerate layer dropped approximately to the middle of the reactor space with each removal.

By comparing the solids content of the water discharged from the reactor with the solids content achievable with known clarifiers, a substantial improvement in the clarification result obtained by the process according to the invention can be observed.

It will be clear to a person skilled in the art that the various embodiments of the invention are not limited to the examples presented but may vary within the scope of the appended claims. For example, instead of an anionic resin, sand, activated carbon, or any suitable material having a specific gravity in excess of the specific gravity of the liquid to be clarified can be used as the suspended particles.

Claims (4)

5,78524 A method for clarifying liquid suspensions, in particular in processes in the woodworking industry, comprising removing fibrous solids from a liquid by introducing the liquid into a reactor (1), binding the solid to a in a bottom-up liquid flow, in which the fibrous agglomerate layer (9) is formed above the fluidized bed (7) formed of finely divided particles so as to bind fibrous material from the liquid flowing through the layer and the clarified liquid (11) accumulates on top of the layer; Method according to Claim 1, characterized in that the liquid to be clarified is circulated from the fiber agglomerate layer (9) to the bottom of the reactor (1). Method according to Claim 1 or 2, characterized in that floating particles (7) with a specific gravity of 1.1 to 2.0 kg / dm 3 and a diameter of 0.2 to 3.0 hb are used. Method according to Claim 3, characterized in that the suspended particles (7) are an anionic resin.