A method for the purification of contaminated aqueous liquid in a pulp or paper mill

Technical field

The present invention relates to a method for the purification of con- taminated aqueous liquid in a pulp or paper mill.

In pulp mills, cellulose pulp is produced. Within the concept of cellulose pulp, chemical pulp, semi-chemical pulp and mechanical pulp fall. Examples of chemical pulps are soda pulp, sulphate pulp, polysulphide pulp and sulphite pulp. Mechanical pulps may be divided into stone groundwood pulp (SGW), pressurized groundwood pulp (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) and chemi-thermomechanical pulp (CTMP). The raw material for the production of said pulps is one or more lignocellulose materials. Most such materials are wood derived from coniferous trees as well as deciduous trees. Depending on what said cellulose pulps are used for, they are divided into paper pulp (which by volume is the heavily predominant type of pulp), dissolving pulp, plastic pulp, etc. A type of paper pulp (which by volume becomes increasingly significant) other than the ones described above is the pulp the raw material of which consists recycled fibres, i.e., newspapers and other printed material. This fibre material

is slushed in a liquid so that a fibre suspension arises, which, after a plurality of purification steps including de-inking in one or more steps, is supplied to a paper mill.

In the paper mills, paper of varying types and different qualities are manufactured. Also paperboard is included in the concept of paper.

In pulp production as well as in paper production, vast amounts of aqueous liquids are used. The process liquids consist of everything from entirely pure liquid, i.e., fresh water, the amount of which being tried to be minimized, if possible down to zero, to heavily contaminated liquid. Said liquids may be named differently depending on in which position they are used in the pulp and paper production. In the bleaching of pulp, for instance, bleaching liquor is used and after the bleaching, the pulp is washed with a more or less contaminated liquid and the arising liquid is usually called spent bleach liquor. In the production of chemical pulp, countercurrent washing is usually used, i.e., the washing liquid is directed in coun- tercurrent vis-a-vis the pulp fibre flow, involving that the pulp after the last treatment step encounters an entirely pure washing liquid or an insignificantly contaminated washing liquid. For a long time, the aim has been to fully close the liquid loop and let the finally arising washing liquid, which is heavily contaminated, follow the spent cooking liquor to the evaporation stage. However, this has no yet be managed completely, but there is a need to open up the closure in one or more positions and discharge a part of the process liquid (which may be denominated washing liquid or spent washing liquor or only spent liquor) to recipient or to a purification plant on the way to recipient. In the production of mechanical pulp also, it is now common to bleach the pulp. This may be made in one or, as is usual for chemical pulp, in more steps. As in the case described above, spent bleach liquor or washing liquid arises, which has to be taken care of in some way. When the raw material for the pulp production is wood, the initial treatment of the logs and the finely divided wood gives rise to contaminated process water, and this has to be taken care of in some way. Neither in the production of paper in a paper mill is it possible to use a totally closed process as to the liquid flow. There is a need to discharge certain amounts of process liquid in one or more positions. Said liquids may have different denominations such as waste liquid, waste water, white water, etc.

In pulp as well as paper mills, at least the major part of the liquid flows discharged from the processes is supplied to some purification plant, for instance a biological purification plant. In such a plant, a microflotation step may be included. Settling steps are usually included, and the proper biostep by activated sludge may be carried out either aerobically or anaerobically. The liquid supplied from such plants to recipient has a limited degree of contamination.

The purification method according to the invention is applicable to any contaminated aqueous liquid, including the waste liquids exemplified above. The invention is used as required, i.e., for individual liquid flows in certain positions as well as for liquid flows brought together, including the collected liquid flow, or part thereof, that leaves the biological purification plant on its way out to the recipient. In those cases where a pulp mill and a paper mill are combined, it is usual to have a common purification of a large number of flows of contaminated liquids, for instance in the form of one or more biological purification plants. In these cases as well, it is fully possible to use the purification method according to the invention in different positions.

Background art

Below, it is described how traditional purification of process and waste liquids may be realized in an integrated combination, i.e., a pulp mill plus a paper mill. In the pulp mill, stone groundwood pulp as well as thermomechanical pulp are manufactured from fresh wood. Furthermore, waste-paper pulp is manufactured from newspapers and other printed material. Without intermediate drying, all pulps are supplied to the paper mill, in which different types of paper are manufactured in different paper machines. The particular pulps included in the pulp furnish, and the quantitative relationship between the same, is determined by the type and quality of the paper produced.

The purification plant comprises two biological purification steps where activated sludge is utilized, biostep 1 and biostep 2. Before biostep 1, there is a microflotation step followed by a cooling step. The biostep consists of two aeration tanks and a subsequent settling tank. Process liquids from two lines for the production of thermomechanical pulp are

supplied to a microflotation plant. By microflotation is meant that a very large number of small air bubbles are brought to rise up through a volume of liquid to the liquid surface. During the upward travel of the air bubbles, solid material present in the liquid is lifted and conveyed. Said solid material, which is present in the form of foam, is scraped from the liquid surface and is transported under the denomination flotation sludge to a sludge-collection tank. In order to facilitate the flocculation of solid material, clay, for instance of bentonite type, and some polymer are added to the arriving liquid flow. The liquid flow exiting the flotation plant has a temperature of approx. 60 °C, and in a cooling step, for instance containing plate heat exchangers, the temperature of the liquid is lowered to approx. 30 ⁰ C. As cooling liquid, water from surrounding recipient is used. On its way to the two aeration tanks mentioned above, nutrient salts are added to the liquid. By nutrient salts or nutrients is meant primarily nitrogen and phosphorus. Said elements are added to the liquid as a mixture in the form of granules. The granules also contain some other nutrients including tracer elements. In order to obtain the correct relation between, among other elements, nitrogen and phosphorus, where nitrogen is dominating, an aqueous solution containing a nitrogen compound is also added. Each tank has a volume of several thousand cubic metres and a depth of approx. 6 m and is completely mixed. Aerobic biological purification is concerned, and hence the liquid is oxygenated with a large excess of air. The air is introduced into the liquid by means of a plurality of blowers. The air from the blowers is distributed through thousands of membrane aerators placed on the bottom of the tanks. From the aeration tanks, the liquid is supplied to a settling tank. This has a volume that is on a level with the aeration tanks. As the name implies, the solid compounds in the liquid in said tank fall to the bottom thereof, from where the solid compounds are removed under the denomination biosludge. This material flow is divided into two parts, a major portion that is returned to the two aeration tanks and a minor portion that is supplied to the sludge-collection tank mentioned above. The purified, remaining liquid is supplied further to additional purification steps, which is evident from what is mentioned further down in the text. As a measure of the degree of purification of waste liquids, i.e., contaminated aqueous liquids, the concepts COD reduction and BOD reduction are commonly used. COD is an abbreviation of

"Chemical Oxygen Demand" and BOD is an abbreviation of "Biochemical Oxygen Demand". In the case described above, the first-mentioned reduction is 80-90 %, while the last-mentioned reduction is 90-95 %.

The second biological purification step includes three aeration tanks, two initial ones having a volume of several thousand cubic metres each and a water depth of 5 m (which may be run in parallel or in a series) and a third, having a volume of about six times the volume of each of the two initial aeration tanks and a water depth of 8 m. The biological purification step also contains a subsequent secondary settling tank. Just before this second biological purification step, there is a primary settling tank having a volume that is approximately on a level with the two initial aeration tanks together. After the second biological purification step, and as termination of the entire purification process, there is a step for chemical precipitation, which is effected in a tank that, as to the volume, is somewhat larger than the volume of the primary settling tank. The above-mentioned purified liquid from biostep 1 is normally supplied to the primary settling tank. However, it is possible to skip this step and supply said liquid to the initial aeration tanks in the second biological purification step. Process and waste liquids from the manufacture of stone groundwood pulp and a line for the manufacture of thermomechanical pulp and the production of recycled fibre-pulp in the pulp mill are always supplied to the primary settling tank. An additional contaminated liquid is supplied to this tank from the pulp mill, viz. the one that arises in the wood handling as well as waste liquid from bark presses. As is known, the wood in the trees and logs produced from trees are surrounded by bark, which has to be removed. This is carried out in barking drums or in other barking plants. In order for the bark to get an acceptable thermal value, the water-interlarded bark should be pressed. The pressed and concentrated bark is supplied to a combustion location, for instance a bark boiler, while the squeezed contaminated liquid is supplied, as is mentioned above, to the primary settling plant. Waste liquids, which may be called white water, are also supplied to said tank from all paper machines. In the production of waste-paper pulp, two flotation steps are used and the scraped-off and collected foam or the sludge, which is usually called waste-paper sludge, is supplied directly to the sludge-collection tank. Sludge from the primary settling tank mentioned above is also finally supplied to this tank. This

sludge is usually denominated fibre sludge. The liquid fed out from the primary settling step is brought to pass a cooling tower, where the temperature is lowered from approx. 45 ⁰ C to approx. 32-35 ⁰ C. Next, similarly to the description above, nutrient salts are added to the liquid. Via plug flow, this liquid is supplied to the initial two aeration tanks, where air is brought to flow through the liquid by means of a plurality of blowers. In the inlet part, one of the aeration tanks has a carrier step where approx. 250 m ³ of carriers of chip type (made from plastic material) are used. In that part of the tank, the aeration takes place by a coarse-blowing aerator system. The second tank has an aerator system including totally approx. 1000 membrane aerators. Following said two tanks, also an iron sulphate solution is added to the liquid. This is in order for the bacteria, which make the work in the activated sludge, should get their demand for iron supplied. From said two initial aeration tanks, the liquid is supplied to the considerably larger third aeration tank. In this tank too, the aeration of the liquid takes place by means of a plurality of blowers. Here, the distribution of the air is effected along the bottom of the tank by means of membrane aerators manufactured from silicone. After completed aeration, the liquid is supplied to the secondary settling tank mentioned above. The solid compounds are accumulated on the bottom of the tank, from where the same, under the denomination biosludge, are brought rearward in the purification plant. The biosludge is divided into three flows, one is introduced into the two initial aeration tanks in biostep 2, and the second is introduced into the two aeration tanks in biostep 1, while the third is supplied to the primary settling. The degree of liquid purification in this part of the purification plant is 80-85 % expressed as COD and 95 % expressed as BOD. The liquid purified in the described way is supplied to a concluding chemical precipitation step. An aluminium compound is added to the liquid in an amount of normally 500 ppm. This is made above all in order to decrease the content of phosphorus of the liquid, but simultaneously also the amount of COD and BOD is reduced. Chemical sludge collected on the bottom of the tank is brought rearward in the purification plant and is introduced into the liquid flow supplied to the primary settling tank. It is not absolutely necessary to add said chemical to the liquid, and in case the chemical addition is omitted, the tank is used as an extra settling step. Only aerobic purification is mentioned above.

As has been pointed out above, it also possible to use anaerobic purification, i.e., purification in the absence of oxygen. As usual, both methods have both advantages and disadvantages. The great advantage of aerobic purification is higher efficiency from a purification point of view. Anaerobic purification involves compact plants, and that a combustible gas results and is utilized. Furthermore, the energy required by the blowers in aerobic purification is avoided.

The described purification measures terminate in two material flows, a liquid having a low degree of contamination and a sludge present in the sludge-collection tank. The sludge has a content of solid compounds of approx. 3 %. By means of a plurality of drum separators, the content of solid compounds of the sludge is increased to approx. 6 %. Next, the content of solid compounds of the sludge is further increased by means of filter belt presses to 38-40 %. Because of this, the sludge gives a heat contribution upon combustion. The break point for sludge to give a heat contribution is about a dry-matter content of 36 %. Reject water obtained upon the concentration of the sludge is introduced into the liquid flow the destination of which is the primary settling tank. The sludge is combusted and destructed in a boiler equipped with a fluidized bed furnace. Resulting ash is deposited on land. The final liquid having a low degree of contamination is divided into two parts. About 20 % of the liquid is taken care of and is recirculated to different positions in the pulp and/or paper production. In spite of the liquid not being entirely pure, it will be good enough to be used as process liquid or part of process liquid in some situations. About 80 % of the liquid is ejected to recipient. The colour of the liquid may be compared with the colour of weak tea. The greater the part of said liquid that can be recirculated, the smaller the demand for fresh water.

Summary of the invention

Technical problem

Based on what has been said hitherto, it is easy to realize that there is a need to be able to purify dirty process liquids in an efficient way. This applies to individual dirty process liquids in individual process steps as well as to dirty process liquids brought together that possibly have been subjected to extensive

purification. However, this purification has to be made in an economically justifiable way.

The solution The present invention supplies these demands and solves these problems and relates to a method for the purification of contaminated aqueous liquid in a pulp or paper mill utilizing activated carbon, characterized in that the activated carbon used in the purification is produced on-site at least principally based on bioma- terial naturally occurring in such plants by said material being transformed into car- bon via pyro lysis, which carbon is activated either in chemical or physical way, and that arising gases are removed and supplied to a receiving location, that the activated carbon produced in the described way is brought together with the contaminated aqueous liquid, which is untreated or pretreated, that after a certain time of treatment, the used, contamination-containing, activated carbon is separated from the purified aqueous liquid, and that the used activated carbon is supplied to a combustion location, while the purified aqueous liquid preferably is used in pulp or paper production.

A core part of the invention is that the activated carbon is produced on-site in the pulp or paper mill based on materials that are economical. Examples of such materials are wood, preferably wood classified to be of low quality, sawdust, bark, sludge and other organic by-products, fast growing plants and preferably such that have been harvested in the vicinity of the mill, and cereals, preferably such that have been cultivated in the vicinity of the mill. Even if it is possible to use wood that has been converted into wood chips of the size that is common in pulp production, it is preferred to have wood chip pieces of considerable smaller size and, for instance, have a length of 2 to 6 mm. Various cereals, such as for instance oats, have almost an ideal size.

The transformation of biomaterial into carbon, which takes place at a very high temperature and in the absence of oxygen (pyrolysis), and the following activation of the carbon may be made consecutively or divided into two steps.

Which solution is chosen depends on several circumstances. It is highly preferable to use physical activation of the carbon, i.e., by using water steam at a very high

temperature, since then high chemical costs are avoided, and furthermore, water steam or steam is something that already is available in pulp and paper mills.

The activated carbon may be produced in granule form or powder form and the last-mentioned form is preferred. As has been indicated above, in pulp as well as paper mills, it is important to purify dirty process liquids overall and particularly to a considerable higher extent than what purification techniques known hitherto is capable of. Such a procedure is possible by using activated carbon produced in the described way. In the bleaching of cellulose pulp, for instance, after the end of the bleaching, a heavily contaminated liquid, spent bleach liquor, is obtained. The same arises by the bleached pulp either being washed using a washing liquid, i.e., the washing liquid displaces the spent bleach liquor out of the pulp suspension, or that the pulp is pressed causing a strong increase in concentration, i.e., the major part of the spent bleach liquor is pressed out of the pulp. Activated carbon may be added to this contaminated liquid, and after a certain reaction time, the major part of the contaminating substances have been captured in and on the carbon, and after removal of the contaminated carbon, the purified liquid may be used afresh in the pulp- production process, either in connection with the bleaching step or in any other position. It has turned out that the purer the liquid is that is present in and surrounds the pulp fibres during the bleaching process, the smaller loss of strength of the pulp is caused by the bleaching.

In the processing of waste paper into paper pulp, frequently white water from some paper machine is entirely or partly used as process liquid. For instance, white water is usually used to dissolve the newspapers so that a pulp suspension having exposed pulp fibres arises. Also in other respects during the pulp purification process of this pulp production method, the liquid system is strongly closed. This causes problems with polymeric interfering substances, so-called stickies in the process liquid. If there is a large amount of stickies in the process liquid, a large amount of stickies is obtained in the paper pulp fed out of the factory to the paper machine, and this in turn causes paper produced from such pulp to have a high content of stickies. If the process liquid is purified in some position, and preferably in the position where the amount of stickies is the largest, with activated

carbon, several advantages are obtained in the pulp production as well as in the paper production. Furthermore, the user of the paper avoids several problems. When process liquids are purified in various local positions, usually this is described as kidneys being installed. The technique described above can also be described as a kidney being installed, and it is naturally possible to install kidneys in several other positions than the ones described above.

A very important application of the purification method according to the invention, and maybe the most important one, relates to the purification of the waste water that today is discharged to the recipient in a relatively large amount. This slightly tea-coloured liquid could be seen as harmless, but after all, it has to be ejected to the recipient. The major part of this waste water that can be purified in accordance with the invention, and therefore be recirculated and used, the smaller amount of fresh water is required in the pulp and paper production, and even in Sweden, rich of fresh and sweet water, this item is becoming short in supply in cer- tain places. The great environmental gain that the recipient is loaded minimally adds to this.

The amount of activated carbon added to the contaminated liquid in question is 0,01-10 % by weight, preferably 1-5 % by weight. This amount is naturally depending on several factors, such as how difficult it is to purify the liquid, desired degree of purification, etc. The truly low charges of activated carbon are possible if a part of the impurities already has been removed in the impure process liquid in one or more pretreatment steps.

The activated carbon may be mixed directly into the contaminated liquid, or be mixed into a smaller amount of the contaminated liquid or in a smaller amount of another liquid so that a slurry is formed, whereupon the slurry is mixed into the contaminated liquid.

The liquid flow depends on the purification location/position and may vary within the interval of 1 to 250 m ³ /h. Furthermore, the activated carbon is added at the temperature and pH- value that the contaminated liquid in question already has, which pH- value is allowed to increase during the purification process. If the temperature of the liquid is low, for instance 20-30 °C, in order to facilitate the separation of the contaminated carbon from the purified liquid, it may be favourable

to increase the temperature of the liquid. However, this possible advantage has to be weighed against the energy cost that arises upon heating. Particularly for large liquid flows, the energy cost rises fast. As regards the fact that the pH-value of the liquid rises during the purification process, usually this is only positive. The reaction time, i.e., the time during which the activated carbon is allowed to act, is usually long and in the order of one or a few hours. However, this does not imply any problems, since in pulp and paper mills, there is plenty of reaction vessels in the form of different tanks where the time of flow of the liquid being treated is in the order of several hours. As an example, it may be mentioned that if it is desired to further purify the finally purified waste water that, among other things, has passed the purification steps biostep 1 and biostep 2 described above, it is possible to advantageously add the activated carbon to the liquid flow being introduced into the chemical precipitation tank described above. The time of flow in said tank is several hours. The long reaction times mentioned above apply to activated carbon in granule form. If activated carbon in powder form is used, which is preferred according to the invention, the reaction time may be shortened considerably if this would prove necessary.

The used activated carbon can be separated from the purified waste liquid (or another liquid) by means of any one of the separation methods settling, filtering, microflotation, or by means of vortex cleaners/hydrocyclones. The separated contaminated carbon is supplied to a combustion location, which may be a fluidized bed furnace to which a boiler is connected, where concentrated sludge arising from miscellaneous waste liquids in pulp and/or paper production is destracted. In said destruction, support fuel has to be used, most often in the form of oil, and said contaminated carbon is used as support fuel instead of the entire amount of or a part of the amount of the previously used oil.

In the production of the activated carbon, a large amount of high- energy, combustible gases (hydrogen, carbon monoxide, etc.) are obtained as byproduct. Said gases may be fed to the combustion location mentioned above to be used as support fuel. If the support-fuel demand already is supplied, said gases may be fed to another combustion location, for instance another boiler. Even if the support-fuel demand is not supplied, from an energy point of view, it may be optimal to

feed the combustible gases to another combustion location.

By count in the optimal use of the energy resources described above in the production of the activated carbon on-site, it is possible to lower the manufacturing cost of the substance to a level that allows the substance to be used for indus- trial purification of miscellaneous impure process liquids.

Advantages

The purification method according to the invention has great advantages from an environmental as well as a material point of view. As regards material advantages, it is possible to, as has been mentioned above, introduce the purification method, as a so-called kidney, in different positions in the pulp as well as the paper production. If, for instance, the spent bleach liquor from the bleaching of the pulp is purified and recirculated, it will lead to a quality improvement of the pulp, i.e., the strength thereof becomes greater. Furthermore, it is likely that it is possible to decrease the charge of bleaching agent, which is economically advantageous. The purification described above of any process liquid in the waste-paper processing, which decreases the amount of circulating stickies, results in a quality improvement of the processed pulp as well as of the paper produced from the pulp. The fact that the purification of process liquids by activated carbon results in an increased pH-value of the purified liquid is an advantage in those applications where an alkaline process liquid is required, because then the cost for the alkaline, for instance sodium hydroxide, additive is brought down or saved.

The fact that the contaminated activated carbon as well as the arising combustible gases are taken care of and combusted makes it possible to entirely or partly replace the fossil fuels, for instance oil, which by routine are used in the pulp and paper production. This makes that the discharge of carbon dioxide emanating from fossil fuels is decreased, and generates discharge permits that may be traded on the market. The steam generated in the combustion boiler(s) is generally brought to pass a turbine, which generates electricity. Such electricity is usually denominated green electricity and gives rise to electricity certificates, which is economically advantageous.

In the case that the entire amount or a part of the amount of the traditionally purified waste water, which today is ejected to recipient, is purified, economical as well as environmental advantages are obtained. For each cubic metre of such liquid that is purified and therefore can be returned and be used as process liquid, the intake of fresh water is decreased by one cubic metre. This leads to, among other things, a great energy saving. In Sweden, the temperature of the fresh water that is taken in as process liquid varies during the year between 1 and 20 ⁰ C. The waste water that today is ejected to recipient has a temperature of approx. 35 ⁰ C. The process liquid/white water temperature in the production of, for instance, paper is usually at 54 ⁰ C. If highly purified waste water is recirculated to the paper production, the temperature needs just be raised from approx. 35 ⁰ C to 54 ⁰ C, while in the case that, for instance, in the winter time, fresh water is taken in having a temperature of 1 °C, the energy consumption for raising the temperature from 1 °C to 54 °C will become enormously much greater. The decrease of the demand for the fresh water is, per se, an environmental advantage, and the decrease of the amount of waste liquid/water ejected to the recipient is, if possible, an even greater environmental advantage.

Description of the drawings In Figure 1, a preferred system closure in the purification method according to the invention is shown in the form of a flow chart.

In the Figures 2-5, results are shown from laboratory purification tests of different process liquids that have been treated with activated carbon of the type that is used in the purification method according to the invention.

Best embodiment

In the following, reference being made to said flow chart, a preferred embodiment of the method according to the invention is described, supplemented with certain facts being explained relatively thoroughly, and in conclusion an embodiment example is given.

In Figure 1, it is shown how biomaterial, for instance wood in very finely divided form, is supplied to a reactor 2 via the pipe 1 for the manufacture of

activated carbon. In the figure, there is not shown how, for instance, the wood is finely divided and how the finely divided material possibly is sieved, and that the material is dried before it is transported to the reactor 2. The drying may be made at temperatures of up to 170 ⁰ C. In the upper part of the reactor 2, the decomposition of the biomaterial into carbon takes place at a temperature of 500 to 750 ⁰ C in the presence of inert gas. The reactor may be composed of, for instance, seven stages, i.e., seven relatively low cylinders piled on top of each other. A rotating shaft runs through the centre of the reactor. Each cylinder or stage has a holed bottom, and on each stage at a certain distance from the bottom, there are arms attached to said shaft. Drivers are attached to the arms. Some energy source has to be used to obtain the required high temperature. For instance, a combustible gas may be supplied via the pipe 3. In several of the cylinders or stages, there are nozzles to which the gas is fed and combusted. The finely divided biomaterial supplied the reactor 2 via the pipe 1 falls down on the bottom of the uppermost cylinder, and there it is exposed to the heat described above. The drivers mentioned above push the biomaterial in front of themselves along the bottom of the cylinder until the material arrives to the holes in the bottom, and by virtue of the gravity, the biomaterial, which is about to be transformed into carbon, falls down to the bottom of the subjacent cylinder. In the described way, the material is brought down through the reactor in order to be col- lected at the bottom of the reactor. If the reactor 2 has seven stages, the three highest ones are used to produce the carbon and the four lowest ones are used to activate the carbon. This is effected by water steam being supplied through the pipes 4 and 5. The activation of the carbon takes place at a temperature of 800- 1100 °C. Already in the production of the carbon, pinholes are formed in the carbon granules. By means of the water steam, additional holes are formed in the carbon granules, and the ones that already exist are enlarged and deepened. This causes the reaction surface of the carbon to increase markedly. As regards the weight loss of the biomaterial on its way to activated carbon, this is of 60-70 % during the pyrolysis step, and the transformation from carbon to activated carbon involves that the amount of carbon is decreased by approx. 50 %. These values are not always given but depend on a number of conditions.

The activated carbon leaves the reactor 2 via the pipe 6. This material

flow may be divided into two flows, one flow that is discharged via the pipe 7 and one flow that, via the pipe 8, is mixed into the liquid in question to be purified, which is supplied via the pipe 9. The carbon flow 7 may be supplied to another position in the pulp or paper production where some local process liquid needs to be purified. If activated carbon is manufactured in a very large amount, i.e., in an amount exceeding the demand in the pulp and/or paper mill, the surplus may be sold on the open market. However, normally all manufactured activated carbon is required for purification purposes on-site. If the liquid in the pipe 9 is the traditionally finally purified (usually a part of it) waste liquid that is ejected to the recipient, large amounts of activated carbon are required. The mixture of waste liquid and activated carbon is supplied to, for instance, a chemical precipitation tank 11 via the pipe 10. The time of flow of the liquid in such a tank is several hours. During the travel down to the bottom, the activated carbon adsorbs impurities in the liquid, causing the liquid to become increasingly pure and uncoloured. Substantially entirely uncoloured waste water is returned from the tank 11 to the pulp and/or paper production via the pipe 12. The contaminated activated carbon collected on the bottom of the tank 11 is, via the pipe 13, supplied to a combustion location in the form of a fluidized bed furnace 14. It is suitable that the contaminated activated carbon on its way to the fluidized bed furnace 14 is subjected to some action, for instance pressing, which increases the dry matter content of the carbon.

The reactor 2 is provided with a chimney (not shown in the figure), through which the arising combustible gases are let out and supplied to the furnace 14 via the pipe 15. Via the pipe 16, said furnace is also provided with the sludge mentioned above, which is to be combusted and destructed. The sludge may be mixed up with cereals, for instance oats, with the purpose of increasing the fuel value of the sludge. According to traditional and prior art, it is necessary to add a fossil fuel as support fuel. This fuel is usually oil, which is supplied to the furnace 14 via the pipe 17 and the branch pipes 18 and 19. Said furnace is connected to a boiler 20. Flue gases formed in the combustion of the described material are brought to pass a large number of tubes containing feed water, which is evaporated into steam of the intended pressure and temperature. Cooled flue gases are removed from the boiler 20, via the flue gas duct 21, to a possible chimney for discharge into

the atmosphere. The solid residual product in the combustion, i.e., ashes, is removed from the combustion location and is deposited on land. If the amount of combustible gases and the amount of used, contaminated carbon is sufficiently large, the oil supply via the pipe 17 may be entirely stopped. The system may also form a surplus of fuel, and if so, the fuel surplus would be to be transported to another combustion location also provided with a boiler, and there replace another fuel, usually oil.

It is stated above that the reactor 2 consists of seven stages. The number of stages may be both fewer and more (all the way up to, for instance, seven- teen). In this reactor, the pyrolysis step is followed directly by the activation step. This does not need to be the way, but said steps may be carried out entirely separated, i.e., the pyrolysis is carried out in one reactor and the activation is carried out in a subsequent reactor. The reactor or reactors may be formed in many different ways. For instance, the reactor does not need to be in the form of an upright cylinder shown in Figure 1, but have a long narrow, lying cylindrical shape, similar to a lime sludge reburning kiln.

In Figure 1, it is shown that the mixture of activated carbon and waste liquid is supplied the chemical precipitation tank 11 from above to the centre thereof. Supplying the mixture in the centre of the tank (the tanks are frequently cir- cular) is expedient and preferred. However, the supply from above is not preferred, but the best is that the mixture is supplied from below and via an ascending pipe in the centre of the tank. The proper mixture of the activated carbon into the waste liquid may be effected in several different ways, which has been mentioned above.

The use of the tank described above as a reaction location/volume for the activated carbon is in no way imperative, but any container of sufficient size may come to use. hi the purification plants for miscellaneous waste liquids used today, there are many tanks or tank-like containers that can be utilized. In case the purification method according to the invention is utilized as a so-called kidney in some process step, existing spaces are primarily utilized, but if no such exists, some type of container has to be constructed. In such cases, it is likely that it is possible to bring down the reaction time from the order of an hour to some tenths of an hour, which limits the required reaction volume correspondingly.

Example 1

Samples of impure process liquids were collected in four positions in the integrated combinations described above. Each process liquid was divided into lots of 200 ml and each lot was placed in a beaker of appropriate size. In each series, several different amounts of activated carbon were added, among which the amounts of 0, 1, 2, 3, 5 and 10 % by weight are particularly mentioned. Initially, this was stirred down into the liquid manually using a glass stirrer. The activated carbon used is of the quality intended to be used in the purification method according to the invention described above. The carbon powder is produced from biomaterial and activated by steam, and is sold under the trade name NORIT®HB Plus, having the following general properties.

Iodine number 1000 - Molasses number (EUR) 260 -

Methylene blue adsorption 19 g/100 g

Surface area (B.E.T.) 1050 m2/g

Apparent density (crushed) 350 kg/m ³

Particle size Di ₀ 4 μm " D ₅₀ 18

D ₉₀ 90 tt

Ash content 7 % by weight

Moisture content (in packed form) 10 "

The beakers were allowed to rest for an hour at room temperature.

Next, the contaminated activated carbon was filtered off from the purified liquid. The filtering was made in a suction bottle provided with a filter medium in the form of glass-fibre paper of the type Munktell MGA-413005. Ocularly, it could be seen that the liquid samples became clearer and clearer with increasing addition of acti- vated carbon.

Each liquid sample was analysed in respect of colour, COD, conductivity and pH.

The colour, expressed in PtCo, i.e., platinum units, was measured by a spectrophotometer of the type XION-500 (dr. Lange no 1071693).

COD, which stands for "Chemical Oxygen Demand", expressed in mg/1, was analysed according to the ampoule method dr. Lange LCK 514. The same type of spectrophotometer as the one mentioned above was used.

The conductivity, expressed in mS/cm, was analysed using a meter of the type WTW LF 197-S.

The pH-value is such a well-known parameter that it does not need to be described. Obtained measurement results are seen in Figures 2, 3, 4 and 5.

A sample of impure process liquid was represented by a spent bleach liquor collected in a TMP line in the pulp mill described above. The bleaching step consists of an alkaline peroxide step, and the bleaching is carried out at a high pulp concentration. Hence, the pulp suspension is pressed just before the bleaching step and the pressate or filtrate is brought rearward in the pulp production process, and after the bleaching step, washing liquid is added to the pulp suspension so that the pulp concentration is lowered considerably. Next, the pulp suspension is pressed once again, and it is the pressate or filtrate formed in this way from which samples were collected. A second sample of impure process liquid was collected just before biostep 1 in the combination described above.

A third sample of impure process liquid was collected just before biostep 2 in the combination described above.

A fourth sample of impure process liquid was collected after biostep 2 in the combination described above.

In Figures 2-5, said process liquids are indicated by the following symbols:

▲ A = spent bleach liquor

X x = to biostep 1 • • = to biostep 2

■ ■ = after biostep 2

As is seen in Figure 2, the colour can be reduced to almost zero for all process liquids. For the process liquids to biostep 2 and after biostep 2, a charge of activated carbon of only 1,5 to 2 % was required for the colour to disappear at an ocular inspection, i.e., by the human eye. For the spent bleach liquor and the process liquid to biostep 1, the charge of activated carbon has to be increased to 5 % to obtain approximately the same purification result.

As regards the reduction of COD, as is seen in Figure 3, it becomes almost complete already at a charge of 1 % of activated carbon for the process liquid after biostep 2. The process liquid to biostep 2 is also relatively easy to pure. The reduction of COD for spent bleach liquor amounts to just below 50 %, and it does not help to increase the charge of activated carbon to above 1 %. It is possible that the COD reduction of the spent bleach liquor can be improved further by means of a supplementing purification step wherein some chemical is added. The process liquid to biostep 1 is the most difficult one to pure at low additions of activated carbon, however, the purification is improved at increasing additions of activated carbon.

The pH-value of the purified liquid rises in all cases with increasing addition of activated carbon, which is seen in Figure 4. In most cases, this is something positive and may in certain cases represent an important cost saving. We are not entirely sure about the reason for the increase of the pH-value, but probably acids present in the process liquid are adsorbed by the activated carbon and/or the ashes included in the activated carbon have alkaline properties, i.e., have hydroxide ions that are dissolved in the liquid.

In Figure 5, it is shown how the conductivity of the process liquid increases with increasing addition of activated carbon. An increasing conductivity is not something desirable, but since the increase is very moderate, it will not cause any considerable problems.