In the manufacture of paper pulp large quantities of COD are obtained, and also higher carbohydrate compounds and lignin, which are difficult to decompose in a biological cleaning process. When an integrated production plant increases its production of paper, cardboard or the like, therefore, a higher production of pulp is required.

Increased pulp production results in increased load on the cleaning equipment of the plant. In a heavily-loaded biological cleaning apparatus, normally in the form of a biological active sludge plant, the extension alternative is often extremely expensive and the cleaning effect is not satisfactory since the easily decomposed substances are still easily decomposed whereas as the substances that are already difficult to decompose accumulate. Thus, upon increased production of paper pulp, the load increases on the biological cleaning process comprising a biological active sludge plant. To reduce this load, and because said compounds are difficult to decompose, the biological active sludge plant is supplemented by an evaporation plant having falling film evaporators for processing a part of the waste water or spent liquor from the paper pulp process. Such an evaporation plant comprises a plurality, e. g. seven, series-connected evaporation apparatuses, also termed effects.

One problem that arises in such an evaporation apparatus is that hard deposits, primarily in the form of calcium oxalate hydrate, CaC204H20, are formed on the heating surfaces of the heat exchanger. The wood contains both calcium and oxalate which combine to form calcium oxalate during the pulping process. Calcium oxalate and its hydrate CaC204H20 are insoluble in both cold and hot water. The water-insoluble, hard deposits containing oxalate, so-called scaling, deteriorate the heat transfer and must therefore be removed in some way in order to restore at least a large part of the full degree of efficiency of the heat exchanger.

The internal heating surfaces of an evaporation apparatus are cleaned from insoluble deposits containing oxalate by acid washing with hydrochloric acid or nitric acid.

However, acid washing must be performed at relatively short intervals and therefore causes many undesired shutdowns during a specific period of time. Acid washing constitutes an investment and thus increases the capital investment in the plant. It entails relatively high costs for chemicals and the cleaning is relatively expensive to perform. It is associated with handling waste material dangerous to environment which in itself is never desirable and which results in extra running costs. It is also unfavourable from the working environment aspect and does not produce fully acceptable results within a reasonable period of time.

The object of the present invention is to provide an improved method of cleaning an evaporation apparatus from deposits containing oxalate that are difficult to dissolve, in order to essentially reduce the problems and drawbacks mentioned above in conjunction with acid washing.

The method in accordance with the invention is characterized in that it comprises the following steps: (a) stopping the evaporation process and emptying the vessel of its spent liquor contents, (b) rinsing the heating surfaces clean from residue of spent liquor with water and emptying the vessel of its contents of the mixture of water and spent liquor thus obtained, (c) supplying heat to the vessel to raise the temperature therein to a predetermined minimum value, (d) causing gas to flow through the vessel to remove moisture therefrom until a dry environment is obtained in the vessel so that the deposits containing oxalate become dry and assume said minimum temperature due to said supply of heat, whereby the deposits fracture into brittle particle structures, (e) discontinuing step (c) and step (d) and causing liquid to circulate through the vessel to remove the particles from the heating surfaces, and (f) drawing off the mixture of liquid and particles from the vessel.

In a preferred embodiment of the method in accordance with the invention the gas in step (d) is ambient air which is drawn in through a lower inlet in the vessel, the moisture-absorbing air flowing out through an upper outlet by virtue of its increased temperature. It is preferable for the heat supplied in step (c) to be supplied via said heat exchanger which is fed with steam having a temperature of at least 70°C so that the temperature in the vessel is increased to a value above 70°C. The time required for step (c) and step (d) is at least about 5 hours, preferably at least about 10 hours.

Normally the desired result is achieved after 12 hours.

The temperature of 70°C is applicable at atmospheric pressure.

The invention will be described further in the following by means of an example of the performance of the new method of removing hard water-insoluble deposits containing oxalate, this example being not intended to limit the invention.

In this example the evaporation apparatus is of falling film type and comprises a vertical vessel with a heat exchanger composed of laminations arranged therein. Spent liquor containing oxalate circulates for evaporation through an outer pipe from the bottom to the top of the vessel and is distributed by means of jets over the top of the heat exchanger to run down along the heating surfaces of the laminations, whereupon the liquid is vaporised and the steam produced is conducted to the next evaporation apparatus in the evaporation plant. The heat exchanger is provided with steam which may be either fresh steam or steam from a previous evaporation apparatus.

When an evaporation apparatus is to be cleaned from scaling containing oxalate that forms a water-insoluble scale on the heating surfaces of the heat exchanger, the evaporation process is stopped for at least the evaporation apparatus in question and the vessel is emptied of its contents of waste water. The vessel is filled to a suitable level with clean water and the water is allowed to circular through the vessel to rinse the heating surfaces of the heat exchanger clean from remaining waste water, which is relatively viscous. All or some of the rinsing water is drawn off and replaced with clean water of lower temperature so that the temperature in the vessel is lowered to a suitable low level that permits inspection of the heat exchanger if

necessary after the initial rinsing. The time required for rinsing and lowering the temperature is about 4-5 hours.

When rinsing is complete and the rinsing water has been drawn off, a lower manhole is opened at the bottom of the vessel and an upper manhole at the top of the vessel. The vessel can now be inspected internally if deemed necessary, and any residue of the viscous waste water can be removed manually from the heat exchanger, using suitable tools, so that the deposits on the heating surfaces containing oxalate are exposed, said deposits reducing the capacity of the heat exchanger.

In the next step steam is supplied to the heat exchanger so that the temperature in the vessel is increased to at least 70°C, preferably at least 100°C. The moist air in the vessel thus becomes hot and water on the outside of heating surfaces of the heat exchanger and other places in the vessel is vaporised. The hot, moist air rises through the vessel and flows out through the upper opened manhole at the same time as new, drier air is drawn in from the surroundings through the lower opened manhole.

This heat treatment is continued long enough so as dry an environment as possible to be obtained in the vessel, and particularly around the deposits containing oxalate on the heating surfaces of the heat exchanger. This normally takes about 12 hours. The dry, hot environment in the vessel causes the deposits containing oxalate to fracture and become brittle so that their adhesion to the heat surfaces is reduced. The hard deposits containing oxalate thus gradually assume a brittle structure of fine particles, some of which fall of their own accord to the bottom of the vessel.

The next step is to remove the particles from the heating surfaces and out of the vessel. The brittle deposits are

preferably initially removed from the surfaces (which may be 20 meter or higher) by mechanical influence in the form of light blows. The force is slight and there is therefore no risk of the surfaces being deformed. The two manholes are then closed and the vessel filled to a suitable level with water, after which the water is circulated through the vessel via a circulation pipe and spray nozzles which distribute the water over the top of the heat exchanger to run down along the heating surfaces, taking with it the particles. The rinsing with water may be performed for about 1 hour and, if desired, may be repeated with a fresh quantity of water after the first quantity, containing most of the particles, has been removed. The final result is heating surfaces so clean that the metal material is completely exposed. The waste water containing oxalate is then conducted to a suitable cleaning plant.

Of course the deposits contain compounds other than oxalate which, to a greater or lesser extent, are difficult to dissolve and which are thus included in the particles containing oxalate obtained through the method in accordance with the invention. Such compounds may be e. g. barium sulphate and various resins. The percentage of insoluble oxalate in the deposits containing oxalate may vary within wide limits and need not necessarily constitute the majority of the insoluble salt content.

As mentioned previously, current methods using acid washing are difficult to perform and it is impossible, using this method, to obtain completely or substantially completely cleaned parallel surfaces up to 10-20 meter high. The present invention, on the other hand, besides producing better cleaning effect on the actual surface, also gives much better cleaning of the whole plant, including nooks and crannies, regardless of the size of the plant. Besides cleaner heating surfaces and

consequently increased efficiency, the method in accordance with the present invention eliminates the investment costs for acid washing. The intervals between shutdowns for said cleaning are also considerably longer with the new method than with acid washing. The expense entailed with the use of chemicals is reduced, as well as the cleaning costs with the new method in comparison with acid washing. Handling waste material dangerous to environment, and the extra expense this entails, are also avoided with the new method, which is also an important advantage over acid washing. Moreover, the new method offers a vast improvement of the work environment in comparison with acid washing.

The reason for the deposit containing oxalate changing from a condition in the form of a coherent, hard, water-insoluble coating to a condition in the form of a loose, brittle deposit is not fully understood but one theory is that the hydrate water in the oxalate is dried off by evaporation in the dry environment at the increased temperature. When the hydrate water evaporates the hydrogen bond of the precipitate which forms the network in the structure and the hardness of the precipitate, cracks. When the network is broken up in this way the binding force of the oxalate molecules is lost and the oxalate thus becomes brittle.

The steam supplied to the heat exchanger should have a minimum water content and for this reason dry steam is preferably used having a temperature of at least 70°C, most preferably fresh dry steam having a temperature of 110°C. The steam requirement is little and a pressure of about 0.1-1 bar is sufficient. Although the example described above is an embodiment that is currently most preferred, the method can be performed in many other ways within the scope of the claims. At least some of the heat supplied to the vessel in step (c) may be supplied, for

instance, by means of the gas in step (d), which gas is dry in order to remove moisture in the vessel while flowing through it, any residual heat being supplied to the heat exchanger. The hot gas supplied to the vessel in step (d) may thus be responsible for the whole increase in temperature.

In accordance with a modified method, the deposits containing oxalate are dried by means of freeze-drying, i. e. by means of decreasing the pressure instead of increasing the temperature.