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FAIR-CT97-3893
Characterisation and control of colloids in paper mills recycling recovered papers |
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Contract No: | FAIR-CT97-3893 |
| Date Prepared: | October 2000 | |
| Source: | Second Annual Progress Report Abstract |
Objectives
The objectives of this project are : To improve knowledge about the origin of the dissolved and colloidal materials (DCM) in order to reduce the sources from the first step of the paper-making chain To investigate various techniques that could be used to remove DCM or to control their detrimental effect To select techniques for DCM removal (or control) able to reduce the DCM content of process water to a significant level.
Activities
The work done is described on a task by task basis below:
Task l: Agreement on a common procedure to characterise DCM. A common procedure to assess the DCM content in water has been defined. In order to check its validity and the correlation between the results obtained by the 4 institutes, it was been decided to apply this procedure to measure the DCM produced by specific raw materials as follows: Raw material for producing dcinked pulp (white grades) End product from a packaging mill (brown grades)
After disintegration of these materials in specified conditions the pulping water has been characterised in term of colloid content using the agreed procedure. Differences in the results obtained by the various partners have led, after discussion, to a repeat of these preliminary trials in a more controlled way. Even so, some differences still occur between the results obtained by the various partners. Nevertheless, the results are in the same range, while the reasons for the differences have probably been identified. Consequently, a final agreement on the procedure to be used has been reached, in terms of initial pulping conditions for brown and white grades, filtration, centrifugation, followed by analysis of conductivity, pH, cation demand, ash content, turbidity, COD (chemical oxygen demand by dichromate method), wire deposition and rotor deposition as appropriate. In addition other standard methods were used without recourse to the inter-laboratory trial. These were drainage capacity, fines fraction, water retention, dye adsorption (methylene blue), dirt, brightness, burst index, stiffness, etc.
Task 2: characterisation of the DCM producing materials The different materials suspected to produce DCM that have been studied are as follows: Crills and fines ) Wood extractives Dry strength additives Wet strength additives Coating binders Water based inks Hydrodispersible pressure sensitive adhesives
The procedure agreed to characterise the DCM has been applied to test these different materials. Their influence on the pulp and on the paper has also been studied in a series of sub-tasks as follows.
2.1. Contribution of wood extractives The aim is to characterise the main sources of DCM which are produced from wood and are soluble in organic solvents. The compositions of the wood extractives in TMP, CTMP and two kinds of recycled paper were studied. After pulping of white, brown papers, TMP and CTMP the water was extracted using MTBE. The content of the extract was analysed applying GC. MTBE extract of the soap which was added for white papers disintegration was analysed too. After pulping, lignans as well as starch were analysed in the water using UV Spectroscopy. Fatty acids, resin acids, glucose and significant content of starch were found in the water after pulping of white papers. On the basis of the results we conclude that a part of fatty acids rises from the soap used. Resin acids, glucose, lignans and traces of fatty acids were found in water after the disintegration of brown papers.
2.2 Wet strength additives Characterisation of DCM from wet-strength papers was carried out using samples of bleached sulphite cellulose with different additions of PAE and UF wet strength additives prepared in the laboratory. The prepared samples were disintegrated and the concentration of DCM in the pulping water was determined. Series of samples of papers with different additions of PAE and 1% of cationic starch were prepared on the pilot plant level. The prepared samples were disintegrated into a slush by applying procedures simulating either brown grades recycling, or white grades recycling (with addition of NaOH and H2O2 during pulping). The DCM concentration in the water was also determined. The main conclusion of this part of the work is that wet strength additives make a very low contribution to the formation of DCM.
2.3 Coating binders In order to characterise DCM originating from coated papers in a recycling process the physical-chemical characterisation of white waters (filtered waters and centrifuged waters) and depositability a study of filtered diluted waters from waste coated paper were carried out. The work programme for this subtask consisted of the study of different parameters on deposition of destabilised dissolved and colloidal material from coating binders. These included: The influence of the coating formulation The influence of coating concentration The influence of the medium characteristics: pH, conductivity and water hardness
2.4 Hydrodispersible adhesives To characterise the DCM originating from dispersible adhesives in a recycling process physical-chemical characterisation of white waters (filtered waters and centrifuged waters) and a depositability study of filtered diluted waters from waste paper containing adhesives were carried out. The work programme for this subtask consisted of the study of different parameters on deposition of destabilised dissolved and colloidal material from adhesives. These included: The influence of the deinking additives and adhesive type The influence of adhesive concentration The influence of the medium characteristics: pH, conductivity and water hardness
2.5 Water based inks In order to characterise the DCM originating from paper and board printed with water based inks the conditions used were those defined in task 1; alkaline pulping for newspapers and neutral pulping for packaging papers. In conventional deinking (alkaline medium), due to the dispersion of the ink's binders, water- based inks produce an over-dispersion of small ink particles. This phenomena results in the release of ink particles from the fibres into the water phase, and consequently contribute to the release of DCM in water.
Two newspaper inks have been studied: an English one and an Italian one to investigate the effect of the ink, unprinted papers have also been tested. The effects of the two inks on cationic demand, COD and NTU turbidity are approximately in the same range. For the plugging test (performed with foams because of the black coloration of the wire itself) using tap waters, the % weight increase were also almost the same.
To compare the contribution of the different DCM sources, the contribution of the inks in microequivalents/g of ink for the cationic demand and in (mgO2/g of ink) for COD have been calculated. These values are rather high for both inks: 574 and 377 microequivalents/g in cationic demand and 1990 and 1370 mg O2/ g in COD. It is also interesting to notice that the two base papers have similar cationic demand values and COD values (40 microequivalents/g and 30 mg/g respectively). Finally, the two inks give high cationic demand and COD, but it must be pointed out that inks are never used as principal constituent, usually the application rate of ink for printing is about .01%, consequently the ink contribution is usually very low. Moreover, it is obvious that large differences in contribution can be found between different marketed inks.
Additional works on the influence of pulping conditions done on the English newspaper. Apart from time, temperature, consistency, only pH and salt conditions seems to have a significant influence on the release of dissolved and colloidal materials.
Two examples of the flexo inks used in packaging have been studied. This included a common one applied on a white top board and a more sophisticated one applied on a coated base paper. In "white grade" conditions the contributions of the inks are different, giving high values of cationic demand and COD. If "brown grade" conditions are applied, the contributions are lower; for one ink, the contribution was insignificant and seemed to be dependent on the type of paper used for application.
In conclusion, various contributions of water based inks to DCM released during pulping have been observed. In general, an alkaline medium gave the highest contribution of DCM, whatever the water based ink. As far as newspaper inks are concerned, their removal in alkaline medium resulted in high CD and COD values. On the other hand the contribution of flexo inks used in packaging, is rather low.
2.6. Dry strength additives In order to investigate the influence of various types of dry strength agent commonly used in the paper manufacture on the release in DCM during recycling the influence of process parameters (such as pH, salt concentration, etc) has also been evaluated using various dry-strength additives. Because the retention of these additives never reaches 100%, some goes directly to the process water. Sufficient amount of such additives remain with waste papers leading to the contamination of the water system. The problem may be serious when the water loop is closed. During the course of the subtask various dry-strength additives were investigated, the paper characteristics were established and the white water parameters were studied.
From the white water two fractions were obtained (filtrate and centrifuged water). The effect on these fractions of various types of dry strength agent (cationic starch, anionic starch, amphoteric starch, poly-acryl-amide) were studied. Total dry content (TDC) and COD were increased by all chemical aids. Colloid content was reduced by cationic starch to the highest degree. Pulp properties were influenced similarly by all agents, while in the case of handsheet properties cationic starch gave the best results. Finally, cationic starch seems to be the best dry strength agent, both for paper and water quality.
The influence of the additive concentration has been studied only with cationic starch. Increasing dosage of cationic starch causes increased COD values, but turbidity (colloid content) had a minimum value at 1% dosage. High doses disturbed the drainage of the fibres. Paper strength showed a saturation curve. Overdosage (over 1.5-2%) of the cationic starch hardly improves the strength, but increases the dewatering time and pollutes the water system.
In the case of anionic starch neutral and alkaline pH, during stock preparation, increased the turbidity of water, but other parameters were not influenced. pH has no significant effect in the examined range (pH 5-8). In case of cationic starch COD values were reduced and colloid content raised by increasing the hardness of the water. In the case of anionic starch the COD increased while turbidity showed a minimal value.
Dewatering time was reduced by 10-15%, but some paper properties were also reduced by the same amount. Influence of conductivity COD showed a minimal value at 0.5 g/l added salt. In case of cationic starch the original low turbidity was increased by increasing the salt concentration, in the case of anionic starch the original high turbidity was reduced. Dewatering time was reduced by increasing conductivity, except at the highest salt concentration (3 g/1). Tensile and stiffness parameters of handsheets were reduced by increasing NaCl concentration, especially in the case of anionic starch.
At increased temperature more contaminants were dissolved from the fibres; this caused higher conductivity, TDC, ash content and COD. SR· and drainage time was reduced at high temperature. There was no significant difference in paper quality at 20·C and 40·C.
2.7 Fines and crills Experiments were designed to determine the influence of the fines on the water characteristics, on sheet forming and on the resulting paper characteristics. The tendency to utilise more and more waste papers causes a shortening of fibre length and an increase in the amount of fines in the pulp. The size of these fines is higher than the colloid size (average size of fines is 0.2-0.4 mm), but their effects on sheet forming, dewatering and efficiency of chemical aids may be comparable with the effects of DCM. In order to analyse the effects of fines and crills on the paper properties and white water quality, the rate of these fines and crills was varied for handsheets made on a circulation sheet forming apparatus. Water, after 15th cycles, was collected and tested according to the methods developed in Task 1. Handsheets, formed after 15 water cycles were also tested. Trials were made with wet broke from a paper-machine producing packaging paper, the crill and fines fraction was obtained from the white water of the same paper-machine by sedimentation.
The added fines have the greatest effect on the drainage properties, the increase was exponential, 40% addition made the drainage time 7.5 times higher, while the SR· value was doubled. These data showed how strongly the runnability of a paper-machine could be effected by the crills and fines accumulated in the process water. The changes in ash content, fine content, SRI value and specific surface were almost linear. The higher negativity of zeta- potential of the fibres causes a higher demand of cationic additives during paper-making. The increased SR· value makes the paper-machine slower. Higher water retention (WRV) means higher energy consumption in the drying section. The increased solvent extract showed the high amount of adsorbed sizing agent on the surface of fines. Effect of added fines on hand sheet properties The density of the sheets increased 3-5% in consequence of the more compact sheet structure.
The added fines reduced the paper quality, the time necessary to get over fix amount of air was increased in the higher degree, which is disadvantageous during filling the sacks produced from that paper. The qualitative change appeared over 30% added fines, similarly to dewatering time. The breaking parameters hardly were influenced by 10% added fines (less than I0%), but 20% added crills reduced them by about 30%. The changes in elongation were not significant. The reduction of the stiffness parameters of the paper (CMT, SCT) were less than 20%, it means that the quality of base paper for corrugated board is damaged in less extent than that of the sack paper.
The added fines increase the amount of contaminants in the water, especially the dry content and turbidity of Fraction I (filtered water). The higher dry content and turbidity of Fraction 2 could result from DCM desorbed from the surface of crill and fines. There is no tendency in changing of the conductivity, so the DCM deriving from the fines was mainly organic. The high turbidity in Fraction 2 (centrifuged water). means, that this organic part consists in high rate of colloid size compound.
2.8 Characterisation of white waters from SCA Lucca raw materials In order to characterise the DCM contribution of the various sorts of recovered papers used as raw material in the SCA packaging mill experiments of depositability were made in order to select raw materials less problematic from the point of view of deposit formation. The disintegration of raw material was carried out in alkaline conditions (1% NaOH).
2.9 Synthesis of task 2 The contribution of each individual source studied was assessed, taking into account its occurrence level in the various recovered papers types used as raw materials. This contribution was expressed in terms of both cationic demand and chemical oxygen demand per weight of the source. The level of occurrence of each individual source has been assessed in the various raw materials. Based on these data the contribution of each source to form DCM in the various raw materials has been calculated. By adding the theoretical contribution of each source a global DCM contribution has also been calculated and compared to the experimental value measured on the water coming from the repulsing of a sample of the considerate raw materials.
The main conclusions of this synthesis are in the case of the brown grades: A rather good correlation between the theoretical value (global contribution) and the experimental value is observed. The surface starch (applied in size press) appears as the main source of COD and CD Mechanical pulp is also a rather important source (essentially for CD)
In the case of the white grades: The correlation between theoretical and experimental values is bad (theoretical value are about 5 times less than the experimental) Some additional work is needed in order to understand the origin of this difference Taking into account the previous restriction, the fibres appear as the main DCM source.
Task 3.: Characterisation of dissolved and colloidal materials in process water of recycling mills
Sub-task 3.2 Characterisation of colloids in a mill producing corrugated board DCM present in a packaging mill process water and the contribution of various types of recovered papers used as raw material was determined in three mills (1 Italian and 2 Hungarian). All of these produce base paper for corrugated board from 100% waste papers, mainly from different kind of packaging wastes. The Italian mill works with low amount of fresh water (cca. 5 m3/t) , the Hungarian mill 1 works with an open water system (cca. 40 m3/t fresh water), Hungarian mill 2. uses a moderate amount of fresh water (cca. 7. m3/t),
The main results of this study showed that the level of DCM content in the water circuits is governed by 2 parameters :
the quality of the recovered papers used as raw material. Indeed, the various raw materials used in these 2 mills were studied concerning their capacity to release DCM in the water. The results clearly show that the level of colloid released is strongly variable depending on the type of recovered paper used. Moreover, even for a defined type, significant differences can be recorded which are probably linked to the heterogeneity of such material.
the degree of water recycling: this study clearly showed that the greater the closure of the water loop, the higher the DCM content in the water circuits.
3.2. Characterisation of colloids in a deinking mill The aim of this subtask was an analysis of process water in a deinking mill, in order to establish the sources and accumulation of DCM at different points. As an addition to the initial programme, the white water obtained from the different pulps used by New Thames Paper Mill were also studied, as well as the effect caused by the use of several additives (retention aids and stickiness controllers) on the DCM present in these white waters.
In order to characterise the DCM present at different points in a deinking mill, the flowsheet of the recycled fibre plant was studied and eight water streams were selected as the objects of the characterisation. These streams were to be analysed several times during a whole production cycle and the test method agreed in task I was followed, presenting minor conceptual differences, as process waters and not white waters were studied. The most interesting streams were also analysed at the beginning of the following period, to check the behaviour of the different parameters as the system was clean again. The working programme for this subtask consisted in the physic-chemical characterisation of the process water in different points of the water circuits and the depositability study of these diverse process waters, using the deposition rotor test developed by the UCM.
The results showed that backwater stream was the "cleanest" one, as it goes to several cleaning and clarifying stages in New Thames paper mill before going back as backwater to the recycled fibre plant.
An interesting result observed was the cationic character of some of the water streams in the RCF plant when the system was clean (result obtained always after shutting down the plant for cleaning the system, which is done every 5 weeks), Samples were taken after the purgomats, which are swimming pool like devices used to clarify water used in the process so that it can be reused in the plant. In order to do so, dirty water is mixed with chemicals (such as cationic flocculants) and air so that any solid material present floats to the top and is removed by the boom rotating around the purgomat. This stage also achieves some agglomeration and removal of anionic trash. The cationic charge of the water after the purgomats suggested that maybe when the system was clean, the water streams did not have as much anionic trash as expected, and so the cationic flocculent additions were too high, and this overdosage caused the change in the water charge.
The most interesting streams in terms of potential stickies formation were Gravity Table I (GT 1), GT 2 and the screw press, as this last stream was the one that showed the highest deposition tendency.
New Thames paper mill uses four different pulps as raw materials: short fibre (hardwood pulp), long fibre (softwood pulp), broke and RCF pulp. The "white waters" obtained from these pulps were analysed in order to evaluate their stickies content and potential depositability, as well as the pulp quality achieved by the RCF plant. The physico-chemical characterisation of the white waters and their depositability studies were carried out.
The results obtained showed that both RCF and broke pulps were the most problematic ones in terms of potential stickies formation. The solids content, turbidity and TSS were much higher in those two pulps (and especially in the RCF one). The "cleanest" pulp seemed to be the long fibre one and it was observed that practically all parameters were much more stable in long and short fibre pulps, especially those analysed in fraction 2 (DCM), as these pulps are purchased from outside suppliers.
The use of different additives (retention aids and stickies controllers) was studied, in order to attain improvements in the behaviour of the RCF pulp in New Thames Paper Mill, especially important aspect when producing paper grades that contain high percentage of recycled pulp. The additives were: fixatives (PAC, polyquat amine, polyquat ammonium salt) and a non- ionic polymer which works as a detackifier. Tests were carried out to check the behaviour of the RCF pulp when the additives were added to it, in order to see if they could be the source of the destabilisation of the DCM. The RCF pulp after the addition was also studied to determine if it was an effective stickies controller.
Task 4: Assessment of removal technologies at laboratory scale At first, an evaluation of the various removal technologies was performed on a "synthetic mixture" including the various DCM sources studied in the framework of task 2. This mixture contains in a defined ratio
After pulping of this mixture according to an agreed procedure, the various DCM removal treatments were applied either on the pulp itself (case of the anionic trash catchers), or on the water collected after a filtration step (case of dissolved air flotation, electro-flotation ultra- filtration).
To assess the ability of various chemicals to fix the colloidal materials onto the fibres two categories of chemicals were tested:
The influence of each of these chemicals on the DCM fixation was characterised by the measurement of the various water characteristics defined in task I (water collected after filtration of the treated pulp).
Task 5: Dissolved Air Flotation
To assess the ability of DAF to remove DCM from the process water the process water collected after filtration is treated. The removal of DCM by Dissolved Air Flotation takes place in two stages. First of all the DCM are coagulated and flocculated by addition of chemical additives. Then, the flocculated material formed is trapped by small air bubble and carried to the surface of the flotation cell where it is removed. Numerous chemicals (or chemical combinations) commonly used in such an application were studied. The dosage of each chemical applied was based on the industrial application. A comparison of the efficiency of the various chemical combinations used, was performed based on a marking system defined for each water characteristics.
The ability of electroflotation to remove DCM from the process water was also investigated. In this case the process water collected after filtration was treated. In the case of electroflotation, an sacrificial electrode is used which produces metallic hydroxides which play the role of coagulant agents. Then the coagulated material (DCM) is removed by gas bubbles produced by the electrodes. The potential of such technique to remove DCM was estimated by assessment of water quality.
The ability of filtration techniques to remove DCM from the process water was also investigated in terms of the various water characteristics. Series of model solutions of DCM which originate from waste paper fibres and primary fibres were prepared. The solutions were filtered through membrane filters and the concentrations of removed DCM were determined. Blends of papers which comprise all components of DCM, originating from waste papers slushing, were prepared too. The prepared blend was slushed and the water was filtered by using different membrane filters. The concentration of removed DCM was determined.
Progress
The work progresses roughly according to what was planed. There is however a small delay mainly due to difficulties encountered in task I (heterogeneous results between the partners at the beginning). However,
Future activities
Continuation of task 4 to 9 (removal technologies)
Task 10 : The most interesting removal techniques will be applied at pilot plant scale according to the most interesting conditions found in the laboratory evaluation. This work is planed for the end of year 2000
Continuation of task 11: dependent on the results of the pilot plant trials
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