Arrangement for and a method of treating pulp, and a method of modernizing a pulp tower

#The present invention relates to an arrangement for and a method of treating pulp, and a method of modernizing a pulp tower. The arrangement and method relate to treating pulp in connection with high-consistency pulp towers, and especially to improvements in discharging pulp therefrom. The invention also relates to modernizations of pulp towers. High-consistency pulp towers are used in the wood processing industry, for instance, for bleaching and/or storage of high-consistency pulp.

#According to prior art, pulp has to be discharged in a diluted form from high-consistency pulp towers. This is because high-consistency pulp cannot be pumped with, for example, a centrifugal pump, which, however, in recent arrangements is practically the only way of conveying pulp from one process stage to another. Therefore, high-consistency pulp (having most commonly a consistency of 20 to 35%) is diluted to at least a medium consistency (of about 10 to 15%) in the bottom part of the pulp tower. This makes the pulp pumpable with a so-called fluidizing centrifugal pump. Preferably, pulp is diluted to a consistency of about 3 to 5%, whereby it will be pumpable with a conventional centrifugal pump. Dilution is effected by introducing either clean water or filtrate from a suitable process stage into the bottom part of the tower and mixing it with the pulp by agitators arranged for that purpose in the bottom part, i.e. a so-called dilution zone of the tower.

#Depending on whether high-consistency pulp towers are used for bleaching or storage, the constructions and appearances of their bottom parts are much different from each other for a number of reasons. Specific to all types of towers is, however, that even dilution is almost unattainable. The reason for this is that high-consistency pulp as well as medium-consistency pulp flows downwards in the tower unevenly. This again is caused by friction between the pulp and the tower wall, which retards the pulp flow so much that between the zone of the diluted pulp in the bottom part and the undiluted pulp in the upper part of the tower there will be formed an arch, which, after having expanded enough, will collapse down to the bottom part of the tower. Since dilution liquid is introduced as an even flow into the tower, the pulp to be discharged from the tower is continuously diluted during arching and, immediately after the arch has collapsed, the consistency will increase to a maximum, whereby the required pulp consistency will remain somewhere between the maximum and the minimum values. In one high-consistency pulp tower the discharge consistency has been established to range from 3.2 to 6.1%. As the pulp is in most cases conveyed from the high-consistency pulp tower to some other process stage, whereby chemicals are mixed with it in pumping or soon thereafter, it is easy to understand that the chemicals dosage per pulp unit cannot be even when the consistency ranges so drastically. Another problem resulting from the collapse of the high-consistency pulp down to the bottom part of the tower may also be difficult, namely it is quite possible that the agitator is damaged by the great volume of pulp falling onto it. In the worst case, the entire process has to be stopped for the repairs of the agitator.

#An arrangement for and a method of treating pulp, and a method of modernizing a pulp tower according to the present invention, are explained more in detail in the following, by way of example, with reference to the accompanying drawings, in which

#FIG. 1 illustrates the bottom part of a high-consistency pulp tower in accordance with prior art,

#FIG. 2 illustrates in a simplified manner the working of a diluting agitator at the bottom part of a high-consistency pulp tower according to prior art,

#FIG. 1 shows an improved prior art high-consistency pulp tower 10 in accordance with U.S. Pat. No. 5,711,600. The bottom part 20 of the tower is provided with a stationary bottom pillar 30, which is preferably cylindrical, although other cross-sectional shapes are also applicable. The upper end of the pillar 30 has, however, been reshaped in comparison with prior art constructions. It is essential to the upper end of the pillar 30 that the diameter of an also stationary parting member 31 disposed therein is at least in one point larger than the diameter of the lower part of the pillar 30. More broadly expressed, at the level of the parting member 31, the cross-sectional area between the parting member 31 and the wall 12 of the tower 10 is smaller than in the bottom area of the pillar 30 below the parting member. In FIG. 1, the parting member 31 is formed of a first section 32, the diameter of which widens conically upwards, and a second section 34, the diameter of which converges conically upwards. In other words, at the point of contact between the first and second sections the diameter of the parting member is at its largest, whereby a throttling is formed between the parting member 31 and the tower wall 12. A purpose of this throttling is to even the downward flow of the high-consistency pulp. Another purpose of the throttling is to separate the bottom part of the tower from the upper part of the tower, as will be explained later on.

#It has to be noted, however, that the term “conical” has been used above and will be also used further below to specify a piece widening, or correspondingly converging, in some direction. So, in practice, the conical parting member is replaceable with, for example, a quadrangular, a pentangular, or a hexagonal jacket. Correspondingly, the term “diameter” may as well refer to a diameter of an imaginary circle calculated on the basis of the area defined by the above-mentioned polygonal jackets.

#FIG. 2 illustrates how the bottom part 20, i.e. a so-called dilution zone, of a high-consistency pulp tower operates in practice. For simplicity reasons, FIG. 2 illustrates only one agitator 40 having its shaft in substantially horizontal direction. The drawing also shows pulp being discharged from only one side of parting member 31 to the mixing or dilution zone of the bottom part of the tower. The shape of parting member 31 serves to exactly mark off the mixing or dilution zone below the largest diameter of the parting member 31 or, more broadly said, below the smallest cross-sectional area between parting member 31 and the wall 12 of the tower 10. Thus, it is the aim of the parting member and its dimensioning that the circulating flow provided by agitators 40 is prevented from rising above the level of the parting member 31. In prior art constructions, the rising of the flow to the upper end of the pillar and even above it caused uncontrolled discharge of pulp from the upper part, the so-called storage part, of the tower to the mixing/dilution zone. Another object of the parting member is that the agitators 40 bring about both a free turbulence and an annular circulation of pulp in the mixing zone of the tower, which free turbulence and annular circulation of pulp, by means of the great difference in both the flow rate and direction, then evenly “cuts” pulp from the slowly downwardly flowing high-consistency pulp to the dilution zone.