WO2004088306A1

WO2004088306A1 - Method for the determination of dissolved and colloidal disturbing substances in a papermaking process

Info

Publication number: WO2004088306A1
Application number: PCT/FI2004/000206
Authority: WO
Inventors: Tiina Sarja; Mari Zabihian
Original assignee: Kemira Oyj
Priority date: 2003-04-02
Filing date: 2004-04-02
Publication date: 2004-10-14
Also published as: FI20030496A; FI119709B; FI20030496A0

Abstract

The invention relates to a method for determining dissolved and colloidal disturbing substances in the pulp, process water or waste water of paper production, the method comprising the following steps : a) forming a water fraction of the pulp, process water or waste water of paper production; b) measuring the turbidity of said water fraction in order to get a first turbidity value; c) after the turbidity measurement, decreasing the pH of the water to a specific pH value; d) measuring the turbidity of water having a specific pH value in order to get a second turbidity value; and e) calculating the turbidity difference between the first turbidity value and the second turbidity value, the turbidity difference correlating with the amount of dissolved and colloidal disturbing substances present in the pulp, process water or waste water. The invention also relates to a method for controlled removal of dissolved and colloidal disturbing substances from a papermaking process.

Description

Method for the determination of dissolved and colloidal disturbing substances in a papermaking process

The invention relates to a method for determining dissolved and colloidal disturbing substances in the pulp, process water or waste water in a papermaking process. Such disturbing substances comprise secondary stickies or microstickies. The invention relates particularly to a method for measuring secondary stickies in the circulation water at a paper mill.

With a more closed circulation water system of a paper mill, one can improve energy management, reduce fresh water consumption and facilitate waste water treatment. However, a closed circulation water system enhances various phenomena such as a tendency to deposit formation, slime formation caused by microbes and foaming caused by gases. These phenomena are due to the accumulation of dissolved and colloidal substances in the circulation water, and they result typically in various problem situations and process failures unless they are controlled or adequately anticipated. There have been efforts to reduce detrimental effects by adding various chemicals, such as e.g. defoaming agents, flocculating and dispersing auxiliary agents, slimicides, and by regulating the pH. However, in most cases, the mechanisms causing these detrimental effects are not exactly known. This usually results in over-dosage and uneconomical use of chemicals.

Process fluctuation is very common in paper production e.g. because of a shift of species, subsequent raw material flow variations and changes of mutual ratios. Fluctuation is also caused by periodic changes in water circulation. These factors often lead to pH and temperature variations. All these causes jointly result in variations of the amount of dissolved and colloidal substances in the circulation water and especially in colloid instability. Uncontrolled loss of stability results inevitably in problematic deposit formation either on the process equipment or the paper proper.

The process is typically equipped with several pH, temperature and flow sensors, which are intended to monitor the developments in each case and to stabilise the run in order to obtain optimal regularity. Consequently, monitoring is not sufficient to anticipate problems. Nevertheless, the actual process problem consists of deposit formation, which is mainly due to colloidal particle agglomeration, i.e. to loss of the stability of colloidal particles. These deposits contaminate the process equipment and deteriorate the paper quality by causing flaws, such as spots and holes. In the worst case, the deposits cause rupture of the paper web and reduce the utilisation degree of the papermaking machine, consequently causing economic losses. Another problem is typically related to the limitations of analytic instruments with regard to the analysis of circulation water, such as problems caused by the presence of fibres, mineral pigments and fine particles.

Turbidity determination has been extensively used for measurement of process water quality and for evaluation of e.g. extractive contents in the pulp water phase. The turbidity of mechanical pulp has been observed to correlate with wood extractives, such as colloidal resin (Holmbom 1997/reference 1; Zabihian 1995/reference 2).

US patent specification 5,437,791 discloses a method utilising pH reduction in the precipitation of dissolved and colloidal substances from the process water of pulp production. In this method, a chemical coagulant is added to the process water before the mechanical separator and precipitation is performed in the pH range 7.5 to 8.5, upon which a portion of the water is evaporated. The pH of the unevaporated concentrate is decreased to the range 1 to 5 and the chemical coagulant is preferably added, resulting in precipitation of flocculants, which are separated, thus yielding purified process water.

US patent specification 6,474,354 discloses photometric measurements at two temperatures. The method performs on-line measurements of colloidal stability in the process water from the woodworking industry. A temperature variation may also cause precipitation, and the method has taken advantage of this.

WO patent specification 96/41245 utilises on-line turbidity measurements in the control of chemical coagulant dosage during water purification. Continuous measurement results in optimal chemical amounts. The turbidity is determined before and after the chemical addition.

By definition, stickies are sticky disturbing substances in the recycled fibrous pulp, which are ultimately entrained by reclaimed paper to the deinking plant. Stickies derive e.g. from self-adhesive envelopes, stickers and the backs of telephone directories. Such stickies are primary ones, given their solid state by their arrival to the deinking plant. Secondary stickies are formed from dissolved and colloidal substances which are present in the process and originate e.g. from chemicals used in the process. Secondary stickies arise when process conditions change, such as sudden changes of pH or temperature. Potential secondary stickies, i.e. "hidden stickies", are dissolved and colloidal substances in the process water, which may form secondary stickies during the deinking process or in the papermaking machine. Stickies form deposits e.g. on the wire, felts and rolls of the papermaking machine.

There is a great variety of methods for determining stickies. Gδttsching and Pakarinen 2000 (reference 3) and Doshi and Dyer 1999 (reference 4) provide an ample description of these. Most of the methods are intended for primary stickies and macrostickies. Macrostickies have a size above 100 μm. There are various precipitating tests (Lenes et al. 2001/reference 5, Konopa 1999/reference 6) for secondary stickies. These tests comprise precipitation of secondary stickies on a surface using e.g. chemicals, the amount or area of the stickies being either calculated (manually or by scanning) or weighed.

Methods for determining stickies are usually time-consuming and require special equipment, or else the outcome will be highly dependent of the person performing the method. Chemical analytic methods give results only for specific chemical substances, and hence the results will not cover stickies as a whole. Moreover, the equipment needed for performing the analysis is available only in very well equipped analysis laboratories. Methods that measure deposit potential have poor reproducibility. Methods for determining stickies by separating stickies with a screen give results only for the amount of macrostickies. There is a very large variety of methods based on classification (Capozzi and Rende 1995/reference 7, Sithole et al. 1998/reference 8, Doshi and Dyer 1999, reference 4/ pp. 25 to 27).

The purpose of the invention is to provide a method for determining stickies that is fast compared with prior art methods and wherein the equipment needed for the determination is simple. The determination method is thus easy to take into use and fast to accomplish. On-line measurement is also feasible. In accordance with the invention, a method has been achieved for determining dissolved and colloidal disturbing substances in pulp, process water or waste water during paper production, the method comprising the following steps:

a) forming a water fraction of pulp, process water or waste water from the paper production, b) measuring the turbidity of said water fraction in order to obtain a first turbidity value, c) after the turbidity measurement, decreasing the pH of the water to a specific pH value, d) measuring the turbidity of the water having the specific pH value in order to obtain a second turbidity value, and e) calculating the turbidity difference between the first turibidity value and the second turbidity value, this turbidity difference correlating with the amount of dissolved and colloidal disturbing substances in the pulp, process water or waste water.

The water fraction in step a) is preferably formed by a preliminary treatment of pulp, process water or waste water in order to separate solid substances. This preliminary treatment is preferably performed by centrifugation. Filtering may also be used.

In accordance with the invention, a method has also been provided for removing dissolved and colloidal disturbing substances in a controlled way from a papermaking process, the method comprising periodical sampling of the pulp, process water or waste water of the paper production and determining the dissolved and colloidal disturbing substances of the samples by

a) forming a water fraction of pulp, process water or waste water from the paper production, b) measuring the turbidity of said water fraction in order to obtain a first turbidity value, c) after the turbidity measurement, decreasing the pH of the water to a specific pH value, d) measuring the turbidity of the water having the specific pH value in order to obtain a second turbidity value, and e) calculating the turbidity difference between the first turbidity value and the second turbidity value, this turbidity difference correlating with the amount of dissolved and colloidal disturbing substances in the pulp or the process water or the waste water,

and when the turbidity difference exceeds a given threshold value, adding a chemical coagulant to the process water or waste water in order to precipitate the dissolved and colloidal disturbing substances, or adding a fixing chemical to the pulp in order to fix the dissolved and colloidal disturbing substances to the pulp.

The water fraction in step a) is preferably formed by a preliminary treatment of said samples of pulp or process water or waste water in order to separate solid substances. This preliminary treatment is preferably performed by centrifugation. Filtering may also be used.

Said paper production pulp is preferably a recycled fibrous pulp and accordingly, the process water is preferably process water from a recycled fibre line and the waste water is preferably waste water from a recycled fibre line. The methods of the invention are applicable also to other pulps and waters of the paper production.

In accordance with the invention, the first turbidity measurement is preferably conducted at a process pH value, which is normally in the range of 6 to 9.

The second turbidity measurement is performed at a specific, lower pH value, which may be in the range of approx. 3 to 5, preferably in the range of approx. 3.5 to 4.5, and especially advantageously approx. 4.

The turbidity measurement can be performed using commercially available turbidimeters, which can be on-line meters or laboratory meters.

For decreasing the pH value, inorganic acids such as sulphuric acid can be used.

In the method of the invention for the controlled removal of dissolved and colloidal disturbing substances from the process water or waste water of paper production, e.g. salts of aluminium compounds, such as aluminium sulphate, chloride or formiate, or various polymeric aluminium compounds, such as polyaluminium chloride or polyaluminium formiate, may be used as the chemical coagulant. Also organic polyelectrolytes, such as a short-chain cationic polyelectrolyte of polyacrylic amide type, which can be used either alone or together with a long- chain polyacrylic amide may be used as the chemical coagulant. Inorganic adsorbents, such as bentonite or talcum, are also usable as the chemical coagulant.

In the method of the invention for controlled removal of dissolved and colloidal disturbing substances from the paper production pulp, e.g. organic polyelectrolytes, which have the typical features of low molecular weight and varying cationicity, may be used as the fixing chemical. The polyelectrolyte consists of a short-chain polyelectrolyte of polyacrylic amide type. The disturbing substances will thus be removed from the process along with the paper.

The method of the invention for controlled removal of dissolved and colloidal disturbing substances from a paper production process can be performed by separating at given intervals a side flow from the main flow of pulp or process water or waste water, the solid material being removed from the side flow, if necessary, and subjecting the obtained water to a first turbidity measurement, followed by decreasing the pH of the water to e.g. 4 and effecting a second turbidity measurement. When the turbidity difference exceeds a certain preset threshold value, a chemical coagulant or a fixing chemical is added to said main flow in order to remove disturbing substances. The amount of chemical addition is determined by the turbidity difference. This method can be performed automatically.

A number of preferred embodiments of the invention are described below.

The determining/measuring method can be used for recycled fibrous pulp and process and waste waters from a recycled fibrous line. In the case of a water fraction having low solid matter content (e.g. the clarified product from a water purifying unit or the like) no preliminary treatment will be necessary. If the water contains solid matter, this needs to be separated from the water. The separating method may be e.g. centrifugation or filtration. If the determination method is applied to pulp, fibres must be separated by e.g. centrifugation or filtration. Hidden stickies are then measured in the aqueous phase of the pulp.

The recommended preliminary treatment method is centrifugation (e.g. 15 minutes, 500 g). The turbidity of the water obtained after the preliminary treatment mentioned in the preceding passage is measured by any commercially available turbidimeter, either an on-line meter or a laboratory meter.

After the turbidity measurement, the pH of the water is dropped to four using sulphuric acid.

The water turbidity is measured at pH 4.

The turbidity difference between the process pH and pH 4 is measured. The turbidity difference = turbidity (process pH) - turbidity (pH 4) indicates the amount of "invisible" dissolved and colloidal substances in the water, these substances becoming visible as the pH or other process conditions change. Such invisible substances are referred to as potential secondary stickies, i.e. hidden stickies.

The measurement can be performed at a laboratory or as a continuous on-line measurement directly in the process. Part of the steps can also be performed on-line while another part is performed at a laboratory.

The turbidity difference is indicated as Netphelometric Turbidity Units (NTU) or as a turbidity difference index without units.

In an especially preferred determination method of the invention, the solid matter of a water/fibre suspension of a recycled pulp plant is separated, yielding water free from solid matter, whose turbidity is measured (1^st turbidity value) at the process pH. Then the pH is dropped to 4 by adding sulphuric acid, and a second turbidity measurement is performed at this pH value (2^nd turbidity value). Then the difference between the first and the second turbidity value is calculated. The method of the invention has the essential feature of measuring the turbidity at two pH values.

The method of the invention allows rapid evaluation of the water or pulp quality during the production of recycled fibres. When measured from the process water, the outcome foresees changes in the water quality, because turbidity measurement at two pH values indicates the amount of disturbing substances that may precipitate eventually. The quality of the raw material varies greatly during the production of recycled fibrous pulp, and this may also affect the quality of the finished pulp, unless the reactions to the raw material variations are rapid enough. This method can be applied e.g. to the control of an internal water purifying unit or the feed of fixing chemicals to the pulp. This allows timely reactions to situations where the raw material entrains disturbing substances in larger amount to the process and the chemicals control requires a change.

Efficient removal of macrostickies from the process can be achieved with various unitary processes, e.g. screens and hydrocyclones. By contrast, removal of micro- stickies and potential secondary stickies is more laborious. They can be removed by chemical precipitation during internal water purification, or they can be fixed to the pulp by chemicals, so that they are removed from the process along with the paper. Using known methods, microstickies and secondary stickies are awkward to measure, and hence one cannot supervise the elimination of these. The new method of the invention allows evaluation of the amount of potential secondary stickies. It also allows for comparisons of the efficiency of the methods for removing secondary stickies.

The invention is described in greater detail below by means of examples, which refer to the accompanying drawings, in which

Figure 1 is a graph showing the effect of the pH on the turbidity of two water fractions at a deinking plant,

Figure 2 is a graph showing the content of the resinoic acids as a function of the turbidity difference,

Figure 3 is a graph showing the turbidity difference as a function of the calcium content,

Figure 4 is a graph showing the results of macrostickies and turbidity difference determined at the mill,

Figure 5 is a graph showing the turbidity difference as a function of latex addition. Figure 6 is a graph showing residual turbidity as a function of the bentonite dose and

Figure 7 is a graph showing the residual turbidity as a function of the polyaluminium chloride dose.

Example 1

Figure 1 illustrates the behaviour of two water fractions at a deinking plant while ' the pH is decreased. With the clarified product of alkaline microflotation, the initial pH is approx. 8.5. As the pH is decreased, the turbidity starts decreasing slightly, which is most probably due to calcium carbonate decomposition. When the pH is decreased to a value below six, the turbidity increases abruptly, resulting in precipitation of pitch and probably of stickies as well. With a pH below 4, there will be hardly any turbidity increase. The clarified product of neutral microflotation has an initial pH of approx. 7.5. The turbidity increases as the pH is decreased.

Example 2

The lower pH value 4 was chosen for the turbidity difference. The resinoic acid and calcium concentrations were determined. The results are shown in tables 2 and 3. The turbidity difference was observed to correlate with resinoic acids, and inversely with calcium, in two different water fractions at the deinking plant. In figure 3, the upper line was obtained for alkaline circulation water and the lower line for neutral circulation water.

Given the correlation between turbidity difference and resinoic acids, extractives belong to the components that precipitate during a pH decrease. In the light of literature (Lyche et al. 1998/reference 9), the solubility of many stickies also drops as the pH decreases, and thus also synthetic stickies not derived from wood will be likely to appear in the turbidity.

Calcium is most likely dissolved so as to form the major portion of solid calcium carbonate, which is introduced in the process along with waste paper, such as printing paper, the filler of fine grade paper and the pigment of coating pastes. Example 3

The turbidity difference was measured at various locations of the deinking plant during a fortnight, and the results were compared to a macrostickie analysis performed during workdays at the plant (the amount of macrostickies in 40 g of final dry deinked pulp). The clarified product of alkaline microflotation free from solids had a pH value of approx. 8.5. A first turbidity measurement of this water was conducted at pH 8.5. A second turbidity measurement was performed at pH 4. The comparison of these two methods is illustrated in figure 4, showing that the turbidity difference (dots) follows the same lines as the macrostickies determined at the plant (bars), especially over the period September 23 to October 1. This allows the conclusion that measurement of the turbidity difference in the process water offers a clue to the pulp quality as well.

Example 4

Latex, which is known to cause stickies and deposits, was added in varying amounts by artificial means to the process water of a deinking plant. As in the preceding examples, the turbidity was measured at two pH values, i.e. 7.5 and 4, and the turbidity difference was calculated. The results are shown in figure 5, indicating that the turbidity difference correlates with latex that has been artificially added to the process water of a deinking plant. The results allow the conclusion that the turbidity difference is an indication of disturbing substances present in the water.

Example 5

The measuring method was also used for the evaluation of the purification result of laboratory deposits in the process water of a deinking plant. Figures 6 and 7 exemplify the results of precipitations using two different chemicals in different doses.

The lower curve in figure 6 depicts the residual turbidity after precipitation with bentonite and centrifugation at pH 7.5. The upper curve illustrates the turbidity of purified samples whose pH had been dropped to four.

For the sake of comparison, figure 7 shows identical precipitation curves using polyaluminium chloride (PAC) as the chemical coagulant. A comparison of figures 6 and 7 shows that using PAC as the chemical coagulant, hidden stickies are precipitated with a PAC dose of 300 ppm or more, because the turbidity does not increase as the pH decreases. By contrast, using bentonite as the chemical coagulant, the turbidity increases by approx. 30 NTU as a result of the pH drop. Consequently, this method allows for better distinction of the functionality of different chemicals.

References

1. Holmbom, B. 1997, Analysis of dissolved and colloidal substances generated in deinking; Wet End Chemistry Conference & COST Workshop, Gatwick, UK, 28-29 May 1997, Pira International, Paper 10, pp. 1 to 12.

2. Zabihian, M. 1995, Storsubstanser pa en LWC-pappersmaskin, dissertation, Institutionen for skogsprodukternas kemi, (Institute for Forest Product Chemistry), Abo akademi, Turku, p. 58.

3. Gόttsching, L. and Pakarinen, H. (eds.) 2000, Papermaking Science and Technology, Book 7, Recycled Fiber and Deinking, Fapet, Oy, Jyvaskyla, pp. 458 to 473.

4. Doshi, M.R. and Dyer, J.M. 1999, Management and Control of Wax and Stickies, 3^rd ed. Doshi & Associates Inc., Appleton, WI, USA, pp. 24 to 31.

5. Lenes, M., Andersen, I., and Stenbacka, U. 201, 11th ISWPC International Symposium on Wood and Pulping Chemistry, Nice, France, June 11 to 14, 2001, ISWPC, pp. Ill to 114.

6. Konopa, J. 1999, Stickies Control: Making the Grade in Newsprint with a Chemically Structured Mineral, 85^th Annual Meeting, Montreal, Quebec, Canada, 26 to 29 January 1999, Pulp and Paper Technical Association of Canada, Preprints A, pp. A219 to A224.

7. Capozzi, A.M. and Rende, D.S. 1995, Particle Management: Innovations in Stickie Control, 1995 Recycling Symposium, New Orleans, LA, USA, 20 to 23 February 1995, TAPPI, pp. 79 to 89.

8. Sithole, B., Filion D and Fletcher, S. 1998, Determination and Classification of Contaminants in Recycled Pulps by Screening Techniques, Progress in Paper Recycling, November 1998, pp. 34 to 44. 9. Lyche, J., Hsu, N.N.-C. and Jenkins, S. 1998, the Source, Nature and Control of Stickies in Deinked Pulp, PTS-CTP Deinking Symposium 1998, Munich, 5 to 7 May 1998, PTS, pp. 1 to 10.

Claims

1. A method for determining dissolved and colloidal disturbing substances in the pulp, process water or waste water of a paper production, characterised in comprising the following steps:

a) forming a water fraction of the pulp, process water or waste water of the paper production, b) measuring the turbidity of said water fraction in order to obtain a first turbidity value, c) after the turbidity measurement, decreasing the pH of the water to a specific pH value, d) measuring the turbidity of the water having the specific pH in order to obtain a second turbidity value, and e) calculating the turbidity difference between the first turbidity value and the second turbidity value, the turbidity difference correlating with the amount of dissolved and colloidal disturbing substances present in the pulp, the process water or the waste water.

2. A method as defined in claim 1, characterised in that the pulp, the process water or the waste water is treated in order to separate solid matter and to form said water fraction.

3. A method as defined in claim 1 or 2, characterised in that the pulp is recycled fibre pulp.

4. A method as defined in claim 1 or 2, characterised in that the process water is the process water of a recycled fibre line and the waste water is the waste water of a recycled fibre line.

5. A method as defined in any of claims 1 to 4, characterised in that the first turbidity measurement is conducted at the process pH value.

6. A method as defined in any of claims 1 to 5, characterised in that said specific pH value is in the range from 3 to 5, preferably from 3.5 to 4.5.

7. A method for the controlled removal of dissolved and colloidal disturbing substances from a papermaking process, characterised in that samples are periodically taken from the pulp, process water or waste water of the paper production, and the dissolved and colloidal disturbing substances of the samples are determined by

a) forming a water fraction of the pulp, process water or waste water of the paper production, b) measuring the turbidity of said water fraction in order to get a first turbidity value, c) after the turbidity measurement, decreasing the pH of the water to a specific pH value, d) measuring the turbidity of the water having the specific pH value in order to get a second turbidity value, and e) calculating the turbidity difference between the first turbidity value and the second turbidity value, the turbidity difference correlating with the amount of dissolved and colloidal disturbing substances present in the pulp, process water or waste water,

and, when the measured turbidity difference exceeds a given threshold value, adding a chemical coagulant to the process water or waste water in order to precipitate the dissolved and colloidal disturbing substances, or adding a fixing chemical to the pulp in order to fix the dissolved and colloidal disturbing substances to the pulp.

8. A method as defined in claim 7, characterised in that said samples of pulp, process water or waste water are treated in order to separate solid matter and to form said water fraction.

9. A method as defined in claim 7 or 8, characterised in that the pulp is recycled fibre pulp.

10. A method as defined in claim 7 or 8, characterised in that the process water is process water from a recycled fibre line and the waste water is waste water from a recycled fibre line.

11. A method as defined in any of claims 7 to 10, characterised in that the first turbidity measurement is conducted at the process pH value.

12. A method as defined in any of claims 7 to 11, characterised in that said specific pH value is in the range from 3 to 5, preferably from 3.5 to 4.5.

13. A method as defined in claim 7, characterised in that said chemical coagulant is polyaluminium chloride, aluminium sulphate, polyaluminium formiate or an organic short-chain cationic polyelectrolyte of polyacrylic amide type, which are usable either alone or together with a long-chain polyacrylic amide.

14. A method as defined in claim 7, characterised in that said fixing chemical is an organic short-chain polyelectrolyte of polyacrylic amide type.