WO2015001186A1

WO2015001186A1 - Method for treating effluents and process waters

Info

Publication number: WO2015001186A1
Application number: PCT/FI2014/050544
Authority: WO
Inventors: Matti Ristolainen
Original assignee: Upm-Kymmene Corporation
Priority date: 2013-07-01
Filing date: 2014-07-01
Publication date: 2015-01-08
Also published as: UY35634A; FI20135723A; FI124769B

Abstract

Method for treating effluents and process waters in a pulp mill or a paper mill comprises: - introducing carbonated white liquor to an effluent or process water (3) containing dissolved calcium, - allowing calcium to be precipitated in form of calcium carbonate, and - separating the precipitated calcium carbonate (7) from the effluent or process water. Carbonated white liquor is obtained in contact of white liquor (2) with gas containing carbon dioxide (1), such as exhaust gas from a lime kiln.

Description

Method for treating effluents and process waters

Field of the Invention The invention relates to a method for treating effluents and process waters of a pulp or paper mill . The invention relates in particular to treating effluents and process waters of a chemical pulp mill .

Background

Effluents of pulp or paper mills are typically treated in waste water treatment plants in order to decrease the amount of the compounds that may be harmful for the environment. The effluents of chemical pulp mills may in addition comprise substances which may be valuable to the chemical pulp making process. Environmental trends and sustainable development are driving factors to find new ways to enhance recyclability of these substances.

Nowadays, chemical pulp mills are often being built in areas and surroundings with very strict environmental regulations. In many areas, a cleaner environment is desired in such a way that the mills produce substances that are less detrimental to the environment. Therefore, it is important to look for new solutions for pulp mills.

In chemical pulp mills, NPE (non-process elements) tend to accumulate in process waters and will cause disturbances in the processes if they are not removed. For example calcium, whose origin is partly the wood raw material of the chemical pulp mill, is known to cause problems. Conventional methods for removing calcium are found to be insufficient, and the level of calcium may still remain high in process waters that are reused in various processes.

Summary

The object of the invention is to provide a method for treating effluents and process waters in a pulp or paper mill. An object of the invention is to provide a method for precipitating calcium from an effluent or from a process water in a pulp or paper mill, especially in a chemical pulp mill, in form of calcium carbonate. Calcium is removed by introducing carbonated white liquor to the effluent or process water containing dissolved calcium (calcium ions). The carbonated white liquor in turn is obtained by bringing white liquor in contact with carbon dioxide to form carbonate ions in the white liquor. The pH of the white liquor, a substance available at the chemical pulp mill as a result of causticizing the green liquor, is alkaline by nature, and its high pH favors the formation of carbonate ions. The contact of the white liquor with carbon dioxide is accomplished preferably by bubbling carbon dioxide rich gas through white liquor. The carbon dioxide rich gas can be pure carbon dioxide, but preferably it is exhaust gas containing carbon dioxide. One preferred source of exhaust gas is the lime kiln where the lime reburning process, part of the "calcium circuit" of the chemical pulp mill, is performed. The lime reburning process yields exhaust gases where the concentration of carbon dioxide is high as a result of decomposition of the lime mud to calcium oxide and carbon dioxide.

Further, the white liquor that is to be carbonated, is preferably oxidized white liquor where the sulfides contained in the white liquor are oxidized. By the use of carbonated, oxidized white liquor, the formation of volatile sulfide compounds, especially hydrogen sulfides, is avoided, if the carbonated oxidized white liquor is introduced to an effluent or process water having an acidic pH. The carbonated, oxidized white liquor is prepared by first oxidizing the white liquor and then bringing the oxidized white liquor in contact with the carbon dioxide.

The carbonated white liquor works as pH modificator and precipitant, raising the pH of the effluent or process water to a range which is favorable for the precipitation of calcium carbonate by the effect of carbonate ions contained in the white liquor. The pH for the precipitation is adjusted for example to be in the range of 9-1 1 . The optimum pH is approximately 9,5.

The effluent or process water that is treated with the carbonated white liquor for precipitating the calcium carbonate is preferably a filtrate, that is, a liquid from which suspended solid matter has been removed. Treatment of the effluent or process water with the carbonated white liquor causes the precipitation of calcium in form of calcium carbonate, which can be separated from the effluent or process water, and the result is effluent or process water with substantially decreased calcium content, below a level that does not interfere with the processes if the process waters or the effluents are to be used later in processes. The removal of calcium is so efficient that the effluent or process water resulting from the treatment can be said to be substantially calcium-free. Other substances can be coprecipitated with calcium carbonate to lower their content as well . Such compounds include phosphorus (also belonging to the NPE), which is coprecipitated with calcium carbonate as phosphates.

The treatment can be performed at a waste water treatment plant of a pulp or paper mill, or in any other location in the mill where there is space for the operation. When carbonated white liquor is used for the precipitation, no extra chemicals are needed, and only the carbonation unit (unit where the white liquor is brought in contact with carbon dioxide) and corresponding piping to redirect the existing material flows need to be installed. The white liquor used for the treatment of effluents and process waters contains sodium which is an essential element in the chemical recovery cycle of a chemical pulp mill . If the treated effluent or treated process water is to be used in such a way in the processes of a chemical pulp mill that they end up in the recovery boiler, the sodium that remains in the process water or effluent will be utilized.

The formed calcium carbonate in turn can be used in the lime kiln of the chemical pulp mill as part of the calcium carbonate feed, in addition to lime mud. Thus, the calcium carbonate originating in the precipitation process will become part of the calcium circuit of the chemical recovery cycle, where calcium carbonate is split to calcium oxide (CaO) and carbon dioxide in the lime kiln and the CaO is used for causticizing the green liquor to make white liquor. Depending on the purity of calcium carbonate and coprecipitated substances, the calcium carbonate can be used instead of lime kiln elsewhere as well. The calcium carbonate, with possible coprecipitated substances, can be totally removed from the recovery cycles of the chemical pulp mill. If the calcium carbonate contains coprecipitated phosphates, it can be used as fertilizer.

Description of the Drawings

The method will be described in the following with reference to the appended drawing, which is a schematic representation of the main steps of the method. Detailed Description

The term "effluent" refers to waste water discharged from a pulp or paper mill which is treated for purifying purposes at a waste water treatment plant of the mill. Effluents may comprise, for example, bleaching effluents and ash leaching purge. In addition, the effluents may contain wood handling effluents.

The term "bleaching effluents" refers to effluents from bleaching unit and washing steps therein. Advantageously bleaching effluents contain bleaching filtrates. Most advantageously bleaching effluents consist of bleaching filtrates. The bleaching filtrates contain acidic and/or alkaline flows. Advantageously bleaching effluents do not contain fibers.

The term "process water" refers to waters that originate from an operation in the pulp mill and are reused in the process, after possible treatment such as filtration etc.. Most commonly process water is water that originates from an operation in the fiber line and is reused. Thus, water issuing from a bleaching step can be regarded as process water if it is reused. "Process water" also refers to effluent that has been purified and is reused in the process after the purification.

The term "oxidized white liquor" refers to white liquor, a fluid formed in the alkali cycle of the pulp mill, from which fluid sulphides have been oxidized. The pulp mill also has cooling waters, sealing waters, reject flows, channel waters, washing waters of the plant, and rain waters, as well as wood processing water. Said waters have typically not been in contact with the pulping process with the exception of wood processing water and some channel waters that originate from process overflows and are therefore in contact with the pulping process. Thus, the emissions accumulated therein are mainly leakages and overflows, occasional emissions caused by apparatus breakages, washing waters of devices, textiles (wires and felts) or containers originating from continuous or batch washings, and leakages from the reject system. These waters can be effluents or process waters, depending on whether they are discharged or reused. The term "filtrate" refers to an effluent from which solid suspended mater is removed by a filtration treatment or a flotation treatment or any other treatment. Thus, the term "filtrate" does not necessarily imply that the solid suspended matter, such as fibers, have been removed by filtration. The term "recyclable purified water" refers to effluent or process water that is purified and is clean enough to be reused as process waters. In this context, the recyclable purified water is especially water from where calcium is removed by the method. The term "calcium" refers to calcium as ions (Ca²⁺) in a solution, unless otherwise stated. In general, calcium is present as calcium ions (Ca²⁺) in a solution, unless precipitated out from the solution as a solid compound, such as calcium carbonate (CaCOs), for example. The term "exhaust gas" refers to a flue gas comprising carbon dioxide. Flue gas is combustion exhaust gas produced by a furnace or a power plant. An example of a flue gas is recovery boiler exhaust gas or lime kiln exhaust gas.

The method can be used especially in a chemical pulp mill where chemical pulp is produced using so-called sulphate or kraft process, that is, in a kraft pulp mill. The operations in a kraft pulp mill are conventionally divided in fiber line, chemical recovery system and wastewater treatment. The chemical recovery system includes an "alkali circuit" or alkali cycle and a lime circulation. In the alkali circuit, spent alkaline cooking liquor (black liquor) is concentrated by evaporation, burnt in a recovery boiler, the resulting smelt from the recovery boiler is dissolved in water to form green liquor, and the green liquor is made to cooking liquor, white liquor, in causticizing. In the lime circulation, the green liquor is causticized by CaO which converts sodium carbonate in the white liquor to sodium hydroxide and precipitates calcium carbonate in the form of lime mud, which is separated from the white liquor, which can thereafter be used for cooking the raw material (for example wood chips) in a digester. The lime mud separated from the white liquor is burnt in the lime kiln to convert calcium carbonate back to calcium oxide, with gaseous carbon dioxide as the other product.

The fiber line operations of the kraft pulp mill include typically cooking, brown-stock washing (washing away the spent cooking chemicals from the pulp), oxygen delignification, bleaching and pulp drying.

According to the drawing, the starting materials 0 of the process are

- gas containing carbon dioxide, e.g. lime kiln or recovery boiler exhaust gas or CO2-gas from storage (tank or bottle), all generally denoted with reference number 1 ,

- white liquor 2, and

- an effluent or process water 3, especially a filtrate. In the first stage I, the white liquor 2 is brought into contact with gaseous carbon dioxide containing gas 1 . This is done by bubbling the carbon dioxide containing gas through the white liquor. This process step, denoted with 4, yields carbonated white liquor. In the process step 4, the calcium contained in the white liquor is also precipitated as calcium carbonate due to the carbonation by the CO2, and the calcium carbonate, which can also contain other impurities such as so-called non-process elements (NPE), can be removed as precipitated material 7 (broad arrow describing the flow of precipitated material). Thus, the carbonation can also be used to make white liquor which is purified for making it more useful as purifying chemical for the treatment of effluents or process waters.

In the next stage II, the effluent or process water is treated with the carbonated white liquor to effect the precipitation reaction between the carbonate and calcium ions in the reaction mixture. The precipitation reaction yields insoluble calcium carbonate. The treatment is denoted with 5. At the same time, the pH is adjusted by the white liquor to the range of 9-1 1 , preferably about 9.5. The temperature of the reaction mixture (effluent or process water and the introduced white liquor) can be set between 25 - 70°C. If the effluent or process water is not at such a temperature that would automatically set the temperature to the required level, the reaction mixture can be heated or cooled. At least part of the heating can be performed by using the heat of the exhaust gas transferred to the white liquor in the preceding stage. In the final stage III, the precipitated calcium carbonate is separated from the reaction mixture using sedimentation, DAF (dissolved air flotation) or any other suitable separation method 6, and it is removed as precipitated material 7, which also contains, besides calcium carbonate, other impurities called non-process elements (NPE) in the chemical pulp industry. The end products F include a purified calcium-free effluent or process water, recyclable purified water 8, which can be used further in the chemical pulp mill, for example as washing or dilution water. The recyclable purified water 8 is directed preferably to such washing operations from where the water ends up in the recovery boiler, so that the sodium in the purified calcium-free water can be recovered. In the whole scale of the chemical pulp mill, the losses of sodium with the white liquor used for the purification are, however, small.

Another end product F is the precipitated calcium carbonate 9, with possible other impurities besides calcium, especially NPEs, such as coprecipitated phosphorus compounds. This precipitate can be reprocessed further for end uses, such as use as fertilizer.

The method can be used to treat effluents and process waters in a chemical pulp mill because white liquor is available in the same mill. No use of extra chemicals is required. Further, the source of carbon dioxide needed for carbonation of the white liquor can be in the same mill, for example recovery furnace or lime kiln. The method can be used also in a paper mill where paper or paperboard is made from fibrous raw material. The method can be used especially in a paper mill where carbonated, oxidized white liquor is available within a short distance, for example when a chemical pulp mill and a paper mill using the pulp are integrated. Thus, the effluents and process waters treated can also be effluents and process waters of a paper mill .

Oxidized white liquor, which is readily available in the pulp mill, is preferably used as the white liquor in the method. The white liquor which has been oxidized for converting the sulphides to less harmful form is usually available in the pulp mill, because the oxidized white liquor is used normally in fiber line operations in oxygen delignification of the pulp in alkaline conditions, which is done after the brown stock washing. Oxidized white liquor can also be used in bleaching. Thus, the carbonated white liquor is preferably carbonated oxidized white liquor. The carbonated oxidized white liquor does not form hydrogen sulphide if the pH decreases.

The white liquor can be carbonated using exhaust gas produced by the pulp mill. The lime kiln is the preferred source of the exhaust gas because of its high carbon dioxide content and the easy availability at the kraft pulp mill . The recommended composition of the lime kiln flue gas (emissions in acceptable limits) is given as example below (calculated from dry gas at 273 K, 101 .3 kPa, corresponding to the oxygen content of 6 vol-% ). The percentage values given are vol-%.

Particles 21 -36 mg/Nm3

CO: 0-60 mg/Nm3

NOx (as NO2) 270-420 mg/ Nm3

SO2 10-50 mg/Nm3

TRS (as S) 5-24 mg/Nm3

CO2 18-20%

H2O 25-30% (of volume of wet gas)

Temperature 250-280°C

The effluent treated by the method is preferably a bleaching effluent, which is typically a significant source of both biological and chemical oxygen demand (COD and BOD). For example chlorine-containing inorganic compounds and organic chlorine compounds from the reactions of chlorine dioxide and/or chlorine may be present in the effluent. Bleaching separates various compounds of lignin from the fibers, which compounds remain in the effluent in form of organic molecules. The method is particularly applicable for treating effluents and process waters that are neutral or acidic, with pH below 9, more preferably with pH below 8, because the white liquor can be used also for raising the pH of the reaction mixture to the suitable level. The bleaching effluents to be treated can be for example acidic bleaching effluents, which typically contain calcium in soluble form. The method can also be used for removing phosphorus by coprecipitation phosphorus compounds from acidic bleaching effluents together with calcium carbonate. As special examples can be mentioned A, D and A D bleaching filtrates. "A" refers to an acid stage of the bleaching unit and "D" refers to a chlorine dioxide stage of the bleaching unit. The acidic effluent or acidic process water can also be an acid filtrate from an acidification treatment, so called acid wash, in the fiber line specifically designed for dissolving phosphorus from the pulp after the oxygen delignification stage before the bleaching.

The white liquor is carbonated to the extent which is required for precipitating the calcium form the effluent or process water. Thus, the amount of carbon dioxide used is adjusted so, that the purified water does not contain exces- sive amounts of carbon dioxide or carbonates.

The method removes efficiently calcium form calcium containing effluents and process waters. The initial level of 273 mg/l Ca can be lowered to values below 20 mg/l according to tests made. Laboratory tests have given 96% reduction of Ca content.

In the following table, results from tests performed with carbonated oxidized white liquor for effluents of a chemical pulp mill are shown. Table 1 . Treatment of effluent with carbonated oxidized white liquor. Effluent was effluent taken after the secondary clarifier of the wastewater treatment plant. Measurements were done for several samples. Clarified = measured from a sample where the precipitated material was settled; filtered = measured from a sample filtered through a 0.45 μιτι filter. Initial Ca concentration of the effluent was 306 mg/l and initial Fe concentration was 0.194 mg/l. clarified filtered

Ca-reduction (%) 90-95 >95

Fe-reduction (%) 20-75 0-35

Thus, the calcium reduction was at least 90 % in all cases.

It was also noticed that the amount of phosphorus in the effluent could be reduced by removing it with precipitated calcium carbonate when the effluent was treated with carbonated oxidized white liquor. By adding carbonated oxidized white liquor to the filtered sample where the measured dissolved phosphorus concentration was initially 1 .64 mg/l, a reduction of 64-67 % could be attained when the final pH after the introduction of the carbonated white liquor was between 9.3 and 10.

All concentrations are expressed as concentrations of chemical elements in solution. After the precipitation and separation of calcium carbonate and possible other impurities, such as coprecipitants, the pH of the purified, calcium-free effluent or process water can again be adjusted to the desired level for the further use of the water.

Claims

Claims:

1 . Method for treating effluents and process waters in a pulp mill or a paper mill, the method comprising

- introducing carbonated white liquor to an effluent or process water containing dissolved calcium,

- allowing calcium to be precipitated in form of calcium carbonate, and

- separating the precipitated calcium carbonate from the effluent or process water.

2. The method of claim 1 , further comprising

- bringing white liquor in contact with carbon dioxide which is used in an amount sufficient to form carbonate ions in the white liquor.

3. The method of claim 2, wherein contact of the white liquor with carbon dioxide is accomplished by bubbling gas containing carbon dioxide through white liquor.

4. The method of claim 3, wherein the gas containing carbon dioxide is an exhaust gas bubbled through the white liquor.

5. The method of claim 4, wherein the exhaust gas is exhaust gas of a recovery boiler or a lime kiln of a chemical pulp mill.

6. The method of claim 1 , wherein the carbonated white liquor is carbonated oxidized white liquor.

7. The method of any of the preceding claims 2 to 5, wherein the white liquor is oxidized white liquor.

8. The method of any of the preceding claims, wherein the pH of the effluent or process water is adjusted by means of the carbonated white liquor to the range of 9 - 11 .

9. The method of any of the preceding claims, wherein the effluent or process water is a filtrate.

10. The method of any of the preceding claims, wherein the effluent is a bleaching effluent.

1 1 . The method of any of the preceding claims, wherein other impurities of the chemical pulp mill contained in the effluent or process water, especially phosphorus, are removed with calcium carbonate.

12. The method of any of the preceding claims, wherein the precipitated calcium carbonate is fed to the lime kiln of a chemical pulp mill.

13. The method of any of the preceding claims 1 - 1 1 , wherein the precipitated calcium carbonate with possible other removed substances is removed from the recovery cycles of a chemical pulp mill.

14. The method of any of the preceding claims, wherein the pulp mill is a chemical pulp mill producing white liquor as part of its recovery cycle of chemicals.

15. Use of carbonated white liquor for precipitating calcium in form of calcium carbonate from an effluent or process water in a pulp mill or paper mill .

16. The use of claim 15, wherein the carbonated white liquor is carbonated oxidized white liquor.