This invention relates to a method of purifying and recovering waste water from a fiberboard manufacturing process.

At the manufacture of fiberboard according to the dry method, for example MDF (Medium Density Fiberboard) , the starting material is lignocellulose-containing fiber material, for example wood chips which are washed, defibered, dried, glued, formed to a fiber mat and pressed in a hot press.

At this process a large amount of waste water is produced, which was separated from the fiber material, for example at chip washing, dewatering screw, gas scrubber a.o. This waste water contains im¬ purities in the form of fiber fragments and other organic matter.

For purifying waste water, an evaporator can be used which con¬ centrates the impurities by evaporation of the waste water. The im¬ purities can thereafter be separated mechanically from the eva¬ porator. The waste water thus purified partially could be re-used in the process, while a large proportion was emitted. The requirements on the purity of waste water have become increasingly severe and, therefore, the purification costs have increased and thereby have deteriorated the profitability of the entire fiberboard manuf¬ acturing process.

The present invention offers a solution of the aforesaid problem by using the contaminated waste water after purification for steam generation.

The characterizing features of the invention are apparent from the attached claims:

The invention is described in greater detail in the following, with reference to the accompanying Figure, which by way of a flow chart illustrates an embodiment of the method according to the invention.

Contaminated waste water from chip washing, press water from the feed screw to the defibering process and scrubber water from the fiber drier is fed through a supply line 10 to an evaporator 11. This eve- porator is designed as a heat exchanger where heating medium in the form of steam of high pressure is directed in a counterflow. The steam used as heating medium is generated in a steam generator 12 and can have a pressure of 12-25 bar, suitably about 16 bar.

In the evaporator 11 the waste water is evaporated and at the same time the heating steam is condensed whereby the impurities are con¬ centrated and mechanically separated. These impurities are dis¬ charged from the evaporator 11 through an outlet 13 in the form of a thickened sludge, the dry matter content of which is sufficient for yielding a positive heat value, which makes its combustion, for example in a roasting boiler, possible.

The waste water evaporated in the evaporator 11 is directed to a steam converter 14, which is designed as a heat exchanger where the evaporated waste water in a counterflow heats the condensate from the evaporator 11, so that it is restored to steam at the same time as the evaporated waste water is condensed. This condensed waste water, which still contains water-soluble impurities, is directed to a purification unit 15 where these solved substances are separated, for example by inverted osmosis. The waste water is purified by this treatment to such a quality, that it can be used as feed water in the steam generator 12.

The steam generated in the steam converter 14 is discharged through a line 16 for being used as consumption steam in the fiberboard manufacturing process. The steam pressure is slightly lower than the steam pressure from the steam generator 12. The outgoing steam pressure, thus, can be 10-20 bar. At a steam pressure of about 16 bar after the steam generator 12, the steam pressure after the steam converter can be about 12 bar. Part of the steam generated in the steam generator 12 is returned via a passageway 17 with a reducing valve 18 directly to the process, together with the steam

in the line 16 generated in the steam converter 14.

The evaporator 11 as well as the steam converter 14 preferably are designed so that in operation contaminated heating surfaces are continuously cleaned mechanically.

The waste water amounts obtained in the process which is supplied through the line 10, normally is of the same magnitude as the demand of feed water for generating steam in the steam generator 12. When the supplied amount of waste water substantially corresponds to the demand of feed water for generating consumption steam, in the entire system for purifying and recovering waste water balance is obtained.

The invention, of course, is not rot restricted to the embodiment described and shown, but can be varied within the scope of the invention idea.