Our own patent SE-C-500 616 previously disclosed a method for carrying out pressurized peroxide bleaching of pulp with a consistency exceeding 8% in a bleaching vessel dimensioned for positive pressure, in which the pulp is fed to the vessel by means of a pump, is heated to a temperature exceeding 90°C, and is bleached with peroxide in an amount exceeding 5 kg/ADMT.

Oxygen gas is formed in conjunction with the decomposition of peroxide. If the output from the above- described bleaching vessel is suddenly stopped, the pressure in the reactor will successively increase as a result of the decomposition of peroxide and the formation of oxygen gas. The problem is not limited to peroxide, but rather of course also applies to other bleaching chemicals with this tendency toward "post-fermentation" at high temperature, such as, for example, chlorine dioxide.

In our own patent application SE-9404299-1 is shown a security system that eliminates the risk that the above-mentioned impermissible pressure limit is reached within the vessel or any part of its peripheral equipment. Among other things is shown that the output scraper, which is arranged in the top of the reactor and cleans in front of the outlet valve as well as the emergency valve, so that plugging up of the pulp in and in front of these valves is eliminated.

The upper part of the reactor as shown in the above- mentioned application, with an output scraper and valves, works well but is relatively expensive to manufacture.

According to the known principle, the output valve and the emergency valve are provided on a separate unit, normally a pipe, above and on the reactor housing itself.

A result of this construction is that the length of the axle of the output scraper, which is driven by a motor that sits arranged farthest at the top, becomes relatively long, which leads to problems both of dimensioning and in the bearings. Furthermore, the amount of material used is relatively large.

The object of the present invention is to provide a construction that solves the above-mentioned problems yet still offers the same advantages when it comes to security.

The object is achieved with a pressure vessel having an output scraper for use with pulp bleaching primarily using peroxide, at high temperature and high pressure, comprising a pressure vessel housing with a first end, and an output arrangement at a second end, whereby the output arrangement comprises an output scraper with at least one scraper arm as well as at least one cleansing member, a driving arrangement, a driving axle, a bearing, a supporting structure for the driving arrangement, and at least one normally opened outlet, as well as, preferably, a second normally closed outlet arrangement, whereby the said normally opened outlet is arranged in the wall of the pressure vessel at the curved gable at the second end.

Brief Description of the Drawings The invention will be explained as follows in greater detail with reference to the attached figures in which: Figure 1 shows a known embodiment for providing a security system in conjunction with a pressurized peroxide bleaching vessel; Figure 2 shows a preferred detailed solution according to the invention of the output end of such a vessel.

Detailed Description A known pressurized peroxide bleaching vessel is shown in Figure 1. The vessel comprises a housing 1 with an input arrangement 2 provided at its lower end, as well as an output arrangement 3 arranged at its upper end.

The pulp is output normally with the aid of an output scraper through a first outlet opening 11 that is connected with a first outlet conduit 14. The pulp is led via the outlet conduit 14 to a so-called standpipe 26, where gas is removed from the pulp. The standpipe 26 is further provided with a spillway 27 that opens into a region that is at least partially surrounded by a wall 28 that is impermeable to fluids.

The upper end of a reactor vessel is shown in Figure 2 with a preferred output arrangement 3 according to the invention. A scraper 4 is arranged to rotate within the vessel 1. The scraper 4 consists of two scraper arms 5 that are arranged using known scraper blades 17 or the like. The scraper arms 5 are securely mounted in a cylindrical sleeve 13, which is mounted at the end of a driving axle 8. Cleansing members 6 are also secured to the cylindrical sleeve 13 (or, alternatively, directly on the scraper blades). The axle 8 is mounted in bearings in a stuffing box unit 9 whose bearings are fixed in the wall 1 of the vessel. The axle 8 is rotated with the aid of a driving unit 7, which is arranged at the other end of the axle with the aid of a supporting structure 10, which is securely welded to the vessel wall 1.

According to a preferred embodiment, the supporting structure 10 comprises a member 10A, which has a U-shaped cross-section and is securely welded to the vessel wall 1, and a mounting plate lOB for the driving arrangement 7. A platform 19 is anchored in the vessel wall 1 using, for example, suitable spacers 18. A first outlet opening is arranged between the lower mount 18 of the platform 19 and the stuffing box 9. The outlet opening 11 is provided in an annular plate that is securely welded to the reactor wall 1. This outlet opening 11 opens via a normally open valve 15A within the outlet conduit 14. In order to eliminate the risk of plugging, the valve 15A is arranged directly against the annular plate. In order to always keep the outlet opening 11 free from any possible pulp plugs, the cleansing member 6 is provided to scrape clean very close to the opening.

In order to ensure secure operation even when the first part of the outlet conduit 14 is plugged up, there is provided a second outlet conduit 16 that opens into the outlet conduit 14 at a point between the inlet to the standpipe 26 and the said output opening 11. A second outlet opening 12, which is also provided in direct connection with the vessel wall 1, is equipped with a normally closed valve 15B, which opens within the outlet conduit 16. The said second outlet opening 12 is also arranged in such a way that the cleansing member 6 eliminates the risk that a pulp plug will arise at this opening.

In what follows, the function of the arrangement will be described briefly.

The pulp is fed according to a preferred embodiment using an MC-Pump to the bleaching vessel 1. The pressure within the vessel is maintained with the aid of said MCB-Pump. Between the pump and the bleaching vessel there is normally a mixer, which is fed with steam in order to raise the pulp temperature. If the vessel is used for high-temperature peroxide bleaching, then the peroxide is normally added in or before the NC-Pump, but it can also be added to advantage at the input end 2, whereby additional negative catalyzing metal contact is avoided. The pulp moves upward within the vessel 1 as a plug flow. When it reaches the output scraper 4, which rotates, the pulp is fed out through the outlet opening 11 into the outlet conduit 14 and thereafter onward to the standpipe 26. If the pressure in the reactor exceeds a certain set-point limit, then there is a control system that opens the security valve 15 so that pulp can be fed out using the by-pass conduit 16 to the standpipe 26.

Many advantages are achieved with the help of the invention. First, the construction leads to a very cost- effective solution. Compared to the prior art, a significantly shorter axle to the scraper is achieved, which leads to direct savings of material, as well as to simplified bearings. Thanks to the new construction, one can additionally provide the supporting structure for the driving arrangement with direct openings, which means that one can easily maintain the stuffing box. The amount of pressure vessel welding in the pressure vessel is, furthermore, minimized since only the two relatively small plates to the outlet openings 11 and 12, as well as the bearing, need to be welded to the vessel as compared with the large circumferential welds (normally approximately 3 meters both where the neck is secured to the pressure vessel and at the upper end of the neck), which are necessary if a large "neck" is used as in the known embodiment. In summary, the invention thus leads to a significantly less expensive solution than the prior art and is additionally easier to maintain.

The invention is not limited to the preferred embodiment shown above but may be varied within the scope of the patent claims so that, for example, instead of an upward-feeding tower, a downward-feeding tower can be used. The invention can of course be applied in connection with all types of "post-fermenting chemicals." Moreover, the invention can also be used to advantage with reactors that are intended for "non-post-fermenting chemicals," such as, for example, oxygen gas, whereby no additional security outlet need be provided.