ONTROL OF DETRIMENTAL SUBSTANCES. Low molecular weight cationic polymers can help

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1 ONTROL OF DETRIMENTAL SUBSTANCES Low molecular weight cationic polymers can help BY D.K. CHUNG, J. CANTRELL AND G. LEFEVRE & G. Lefevre, Nalco Canada Inc. Now with National Silicates Ltd., Toronto, ON. D ETRIMENTAI. SUBSTANCES is a term commonly used to identify a wide range of dissolved and colloidal, anionic and nonionic material which adversely affects productivity and paper quality [ 11. Every mill has its own particular "witches brew" depending on a number of parameters including: The pulp (CTMP, TMP, groundwood, sulphite, kraft) ; 4 Wood species; 4 Bleaching process (peroxide, chlorine, hydrosulphite); 4 Additives (size, defoamers, de-inking chemicals, starches); 4 Contaminants in the furnish (ink binders, hot melts, coating materials); 4 Contaminants in the raw mill water; 4 Process ph. The concentration of detrimental substances varies considerably from mill to mill. It is a function of the rate at which the material is generated and the rate at which it is removed. Removal is effected by evaporating water in the paper machine dryers (detrimental substances remain with the sheet) and by sewering process water. However, because of the increased closure of white water loops, the best way to reduce the detrimental material is by removing it from the process system with the production paper. Nowadays, all pulp and paper mills are under strong pressure 10 improve product quality and at the same lime reduce operating costs. The use of' CTMP pulp, more peroxide bleaching, the use of de-inked and recycled fibre etc. are a few examples we can easily cite here. Unfortunately, all these changes have caused a dramatic upward trend in the concentration of detrimental substances. ODVERSE EFFECTS Detrimental substances are either nonionic or highly negatively charged in nature and have a very small particle size compared to papermaking fibres and fillers. The combination of minute size and high charge make them extremely difficult to retain in the paper web. This leads directly to a build-up in the white water loop, which if not treated, will eventually give runnability and paper quality problems. The unwanted extremely small organic substances will first agglomerate to form pitch-like deposits in the paper machine system and Uhle vacuum boxes. These deposits will then gradually build up and eventually cause runnability problems (breaks, down time for wash-ups) at the wet end of the machine. A build up of detrimental substances in a closed water system can lead to sheet quality problems, especially in those grades using waste or mechanical fibres as part of the furnish. In some circumstances, these substances will adsorb onto the surface of the papermaking fibres and actually interfere with fibre to fibre bonding. This will give problems of surface linting which becomes more prevalent when using the more demanding printing methods, such as four-color heat-set web offset. In other circumstances, the detrimental material will interfere with the activity of other wet end additives such as filler clays, retention aids, wet/dry strength additives. They can retard press dewatering and reduce dryer efficiency. The presence of large amounts of' detrimental substances in mill effluenls is primarily responsible for high BOD and COD levels. Therefore effluent disposal costs can be significantly reduced by controlling the build up of these substances in the machine wet end. OONTROL STRATEGIES In the past, alum was predominantly used for controlling pitch deposition. Some mills still use alum despite the many problems associated with this practice including corrosion, poor bleaching efficiency, and sheet yellowing. Dispersants such as low molecular weight (LMW) sodium acrylate, polyphosphate and naphthalene sulphonate condensate polymer are also used. However, the strategy of keeping the detrimental substances dispersed until they are removed from the system fails when a certain level of white-water closure is attained. In addition, the high levels of BOD and COD in the effluent which result from this practice are difficult and expensive to treat. Finally, this strategy actually increases the concentration of detrimental substances in the wet end of the paper machine limiting the opportunities available with other additives. Eliminating the problem at the source is another strategy. Some mills have removed certain species from their wood supply. Others have added inventory control of round wood or chips to promote bacterial treatment of the detrimental substances [3]. Chemical treatment of the fresh water supply has 68 PULP & PAPER CANADA 94:12 (1993) 4 T403

2 also proven beneficial. Retention aids which retain their activity in the presence of detrimental substances have been designed. This approach, however, fails to provide the benefits associated with the optimum control of the detrimental substances. Other chemical programs have been developed to reduce the concentration of detrimental substances in the wet end by increasing the rate at which they are removed with the paper. Talc and Bentonite clay are used to adsorb the dissolved and colloidal material, followed by a high molecular weight (HMW) flocculant to remove the clay with the paper. Another approach is to treat the system with special LMW cationic polymers. The use of these polymers is the subject of this paper. Coagulation mechanisms: Chemically, coagulants are LMW cationic polymers which have a much higher charge density than conventional HMW fpolyacrylamide flocculants. Coagulants will be readily adsorbed onto the surface of the anionic detrimental substances. The highly charged anionic substances are surrounded by a diffuse cloud with an overall cationic charge which results in like-charge repulsion, Fig. 1. This prevents the particles from coming any closer together than the boundary of tlie diffuse cloud. The addition of a primary cationic coagulant increases the concentration of cations, which acts to shrink the diffuse layer, in a process known as uniform charge neutralization [4]. Reducing the effective radius of the diffuse layer, allows the particles to pack much closer together, and a Electronegative l- - I Particle this effect is called charge neutralization coagulation, Fig. 2. Coagulation is also accomplished by another mechanism described by the patch model [5]. LMW cationic polymers will adsorb readily onto anionic surfaces in patches as shown in Fig. 3. The density of cationic charge on the polymer chain is more than sufficient to neutralize the anionic charge thereby producing a localized cationic patch. This positively charged patch is attracted to uncovered negative areas on other particles which leads to coagulation. The coagulation rate of this mechanism is greater than that produced by charge neutralization with a simple coagulant. In addition, charge neutrality is not required with the approach. The optimum density of cationic patches is often achieved with only a slight reduction of the net anionic charge of the system. The agglomerated particles formed through this coagulation process are not shear sensitive and will reform if subjected to mechanical shear. Once the colloidal material has been coagulated it is probable that the process system will still have an overall cationic demand. Therefore a secondary addition of a HMW polyacrylamide flocculant can be added late in the system to provide the degree of flocculation necessary to optimize retention. Therefore, by a process of neutralizing the colloidal material and fixing it to the fibre in the form of micro-particles, the optimum conditions for retention are created. This in turn allows the removal of the colloidal material from the system in the paper web, reducing the degree of Effective radius \---i contamination of the whitewater giving a cleaner system both in terms of charge and the potential for deposition. Measure and control: It is vitally important to be able to measure and control the degree of charge neutralization taking place so that both retention and drainage can be optimized. There is no doubt that if the charge of the wet end is stable and controlled to the correct degree, then it will automatically achieve optimum fibre and ash retention. A typical charge neutralization curve is shown in Fig. 4. This shows the effect of increasing the addition level of cationic coagulant, and the resulting change in charge, as determined using the Nalco colloid titration technique. As would be expected, the charge is consistently neutralized by increasing levels of cationic coagulant and it is possible LO neutralize the charge to such an extent to send the system totally cationic. Our experience has shown there is an optimum level of charge where retention and drainage are maximized without interfering with machine runnability. In general the charge of the wet end should be kept between and eq/ml and should never be allowed to go to zero or cationic. Should any system be allowed to go beyond these limits into the cationic area, then a number of problems will immediately be seen on the paper machine. Once into the cationic state, drainage through the machine wire will be dramalically reduced, as the water molecules will tend to be bound in to the sheet. This results in changes in wet line, increased moisture content into the C ' ++ Rigid layer of counter-ions + (Stern Layer) Coagulant Colloid Diffuse cloud of cations addition T404 4 PULP & PAPER CANADA 94:12 (1993) 69

3 presses, and loss of draw control through the dryers. All of these can give severe runnability problems. In addition, over-charging the process will also result in poor first pass fine, chemicals and ash retention, leading to poor final physical properties of the sheet. There- fore, in order to optimize retention and drainage, we have to monitor and control the correct charge balance in the process system. Case history No. 1: The mill produces newsprint on two fourdrinier machines with a furnish of groundwood aiid high- yield sulphite (HYS). No alum is used. In the past, pitch was controlled with dispersants fed to each pulping process. Eventually, the mill plans to add clayfilled specialty grades to their product mix. Recognizing the requiremellt that ash retention be controlled with reten- anionlc surlace -0.2 L------d Days d -0.7" -0.8 _I_---J-i Days peqlml 0.0 Percent FPR 1 "I Program A +Program B *A+C 1 kglto -0.8 I "; Progpm A,f Prpgram,B, LJ Dosage, kg/ton Dosage, W:on 70 PULP 81 PAPER CANADA 94:12 (1993) 4 T 405

4 tion aids, the mill waited to eliminate the use of dispersants which they felt would interfere with the retention aid chemistry. After the dispersant was turned off, a proprietary low molecular weight cationic polymer was fed to the groundwood bull screen chest and the HYS secondary refined stock chest at 0.2 kilogram/tonne (kg/t). The use of this type of chemistry for pitch control is patented by Nalco Canada Inc., formerly Alchem Inc. (Patent Number Dated ). As shown in Fig. 5 uncoagulated free pitch in whitewater as measured using Allen's method [7] \vas controlled in both the groundwood and sulphite pulps within eight hours ofthe chemical addition (shown as time 0). This level of pitch control was achieved with only a very slight reduction in the soluble cationic demand of the system. Whereas pitch control was accomplished very quickly, the change in headbox soluble cationic demand a s shown in Fig. 6 was very gradual, taking place over several days. The low dosage of the polymer slightly increased the rate at which detrimental substances (other than colloidal pitch particles) were removed from the system with the paper. Due to the enormous volume of the pulp mill and paper mill systems, a long period of time was required for a new equilibrium condition to be reached in response to this small change. Pitch control in this mill has been verified with other tests. The pitch content in the sheet has increased from 0.29% to 0.44%. This in turn has resulted in a 20% increase in the level of natural sizing as measured by the Bristow test. Case history No. 2: This mill produces groundwood specialities using 100% CTMP bleached with hydrogen peroxide. No additives were used for either retention or pitch control. A substantial improvement in opacity was required. The results of the pretrial lab evaluation are shown in part in Fig. 7 and 8. The total dissolved and colloidal substances (TDCS) in this mill varies in direct proportion with the pulp brightness target which is changed according to the grade being produced. With the highest brightness grades, the level can climb as high as 3230 ppm. This level corresponds to a soluble cationic demand of -1.0 microequivaleiits/ml. The headbox sample used for the lab evaluations had a charge of peg/ml. In Fig. 7 the charge neutralization effectiveness of a low molecular weight cationic polymer derived from diallyldimethylammonium chloride (product A) is compared to that of polyethyleneimine (product B). Various levels of charge neutralization can be achieved with about one half the dosage of product A as to B. Through experience, the dosage required in a mill to T peq/ml..- C.D uealml '.. FPR % I...,. I,..,.... percent FPR start start Product A Product C.'.. I V I I I I I I -0.8 ' days achieve the level of response shown would range between 1 and 4 kg/t. In Fig. 8 the effectiveness of the two products at improving first pass retention is compared. In this case product B is more effective than product A. However if a high molecular weight cationic flocculant Product C is used in coii.jimction with product A, substantial improvements in retention can be achieved. Of interest in this figure is the reduction in retention which occurs as the dosage of the low molecular weight product alone is first increased. This effect was noticed for both the product A and product B and is mentioned here because of a curious similarity to the results observed during the mill trial which is discussed next. As a result of the lab valuation, a twostage trial was approved. The first stage involved the pretreatment of the CTMP stock system and the paper mill wet end system with product A. This stage was planned to extend over several days until a new equilibrium level of detrimental substances was established. The second stage would then be the addition of the high molecular weight cationic flocculant (product C) to the headbox approach piping of one of the fourdrinier paper machines to increase fines retention. The results of the first stage of the evaluation are shown in Fig. 9. Product A was fed at 2.0 to 3.0 kg/t to the CTMP thick stock following the bleaching stage. Immediately after the low molecular weight polymer dosage was started, first pass retention dropped from 63% to 60%. This drop could be associated with the initial cleaning up of the system as the new equilibrium condition was being established. As mentioned above, a similar drop in retention was observed in the lab testing. In the lab, the drop occurred as dosage increased. In the trial, the drop occurred as time increased. One problem with lab testing is the difficulty of accurately simulating the recycle of detrimental substances in the white water loops. The correlation between lab and mill trial results ma!. indicate that the effect of recycle in the mill can be simulated in the lab by increasing the polymer dosage. After the initial drop, first pass retention increased steadily over a two-day period to about 68%. This represents a significant five-point net gain over normal performance. Also shown in Fig. 9 is the soluble cationic demand of the headbox stock. A rapid and substantial reduction in charge from to peg/ml resulted from the addition of product A at 3.0 kg/t. Following this period, the rate of peroxide bleaching was increased in preparation for a grade change planned for the next day. This change caused the charge to increase gradually over a 24-hour period. The fact that charge only increased to -0.6 peg/ml indicates the effectiveness of the low molecular weight polymer treatment. Normally, the mill operates with a charge of -1.0 p&ml during the production of paper with this brightness target. Having established a new equilibrium of detrimental substances, Stage 2 was commenced with the addition of 0.5 kg/t of product C, a high molecular weight PULP & PAPER CANADA 94:12 (1993) 71

5 cationic flocculant, to the headbox approach piping. First pass retention improved from 68 to 72%. Headbox consistency was reduced from 1.0 to 0.52% and white water consistency from 0.38 to 0.16%. With this level of dewatering performance, the hydraulic capacity of the fan pump was reached restricting further increases in the dosage of the flocculant. Opacity improved 2.5 points. Parker Print Surf smoothness improved from 3.7 to 3.2. The frequency of paper machine washups has been reduced from every fourth day to every sixteenth day. As a result of these improvements, the chemical treatment program has been continued since this trial. Shortly after, the mill ran a short trial with filler clay to evaluate the improvements in opacity which would accrue with 5% sheet ash. First pass ash retention was 40% with the constraints imposed by the equipment limitations on the dosage of high molecular weight flocculant. Opacity was improved an additional 1.5 points. OXPE RIME WTA L A. Test procedure for first pass retention (FPR): The Britt Jar (or Dynamic Drainage Jar (DDJ)) is used to duplicate paper machine retention with a simple lab test aimed at the effect of attraction or repulsion forces rather than physical entrapment of fines and mechanical entanglement of fibres. In order to separate these factors, the test is conducted under controlled shear conditions (dynamic) to avoid mat formation. Betest pmcedures: 1. Prepare and dilute polymer solutions. 2. Determine stock consistency and adjust ph. The best working range for consistencies is 0.2 to 0.7% (FPR is independent of consistency in this range). 3. Calculate polymer dosages on a kg/t basis. 4. Assemble the Britt Jar with a predetermined screen (70 mesh for retention work). Method: 1. Add stock (500 ml) to the mixing chamber while the impeller is mixing. If the stock is added while the mixer is shut off, a mat will form on the screen giving high FPR values. Do this at three different rpm settings - 800, 1000, 1200 rpm - to an untreated sample to match on-machine retention to that you get in the jar (eg. 55% for a blank). 2. Once you have selected your rpm setting for the test, leave it at this speed for the testing period. (As rpm is increased, retention of fines decreases). 3. When comparing polymer programs, keep total agitation time the same for equal shear. 4. At the end of the set agitation period, drain white water by opening the stock cock (Note: stirrer is still on while you drain!). Discard the first 50 ml you collect as this is a dead volume (untreated white water that fills the bottom unit even before treating). Collect 125 ml in a beaker and then shut off the mixer. The DDJ gives you no information on drainage rates as the flow is controlled by the nozzle size. 5. Measure 100 ml of collected white water and filter through a Buchner fiinnel with a preweighed Whatman No. 41 paper. 6. Remove the filter pad, dry and weigh the residue. This is used to calculate your FPR. 7. Clean the jar by dismantling and washing the screen thoroughly with a water spray to clean the holes. C;alculalioizs: Headbox - white water consistency consistency FPR = x 100 Headbox consistency Notes lo retncmdc.r: 1. Avoid mat formation. This can be caused by lack of agitation, high consistency, small screen size (finer than 200 mesh), height of impeller from screen (no higher than 0.3 cm), and too rapid drainage rate (less than 20 seconds to collect 100 ml). 2. Always add stock solutions with the impeller mixing. 3. Keep total agitation time period constant for all tests. B. Test procedure for cationic demand: The colloid titration technique is used to determine the net soluble colloidal charge of a furnish. It can be run on any stock, water, or additive from the pulp and paper mill. The method involves the titration of a sample with a standardized anionic polymer solution (PVSK) until an excess of the anionic titrant is detected by the color change from blue to purple (TB indicator). Filtration of samples for test: Prepare the samples for charge determination by filtering through coarse filter paper (Whatman No. 4 or No. 41). If filter paper is not available use paper towelling or coffee filters to pass your sample through. All fibrous material must be removed since fibre theoretically has an infinite demand or adsorption for cationic polyelectrolytes. Filter samples regardless if it is a white water sample or thick stock sample. Gravity or vacuum assisted filtering have been observed to give equal results. Use the resources available. Once your samples have been filtered you can leave the sample as long as you want. The soluble charge will not change over time once filtered. Volume required is 10 to 20 ml. It is important that you pre-soak your filter medium in the test solution so that the filtrate will not change charge due to the filter paper itself. 1. Pipette 10 ml of filtered sample into a 250-mL flask, (see filtration procedure). 2. Dilute sample with 100 ml of distilled or deionized water. Tap water contains numerous ionic species which will change the test. 3. Pipette 5 ml of polydadmac solution into the sample. Swirl to mix. 4. Add four drops of TB indicator to the sample. The solution will turn blue. 5. Titrate with PVSK solution until the entire solution turns purple. Record the volume of titrant as A. Ensure good mixing during the titration. 6. Repeat the titration with only 5 ml of polydadmac solution in 100 ml of distilled or deionized water (Step 2-5). t de la plte mecanique, des casses de fabrication un certain nombre de problbmes causes par les Abstract: Paper mills using mechanical pulp, broke or recycled fibre can experience a number of problems caused by the high levels of detrimental substances typically found in these furnishes. Certain low molecular weight cationic polymers have been found particularly effective at eliminating these problems. Recent case histories are given which demonstrate the benefits of this program including excellent pitch control, and improved activity of other wet end additives such as retention aids. An example where improved fines and filler retention resulted in improved opacity is discussed. Reference: CHUNG, D.K., CANTELL, J., LEFEVRE, G. Control of detrimental substances. Pulp P ap Can 94( 12): T (December 1993). Paper presented at the 1989 Pacific Coast- Western Branches Joint Conference of the Technical Section, CPPA, at Whistler, BC, on May 24 to 27, Not to be reproduced without permission. Manuscript received May 4, Revised manuscript approved for publication by the Review Panel March 26, Keywords: IMPURITIES, REMOVAL, PAPER MILLS. 72 PULP & PAPER CANADA 94:12 (1993)

6 1 Record the volume of PVSK titrant used as B. Typical value is 5 2 mls PVSK (not 5.0!). This is your blank. Calculation of snnzple colloid rlinrge: ml A - ml B - ml sample Cationic charge demand (peg/ml) Notes: 1. If the volume of A is less than 2.0 ml repeat the test using a 10 ml cationic spike (polydadmac solution). Remember to retitrate the blank B volume also. 2. If the volume of A is still less than 2.0 ml with a 10 ml cationic spike, repeat the test with a 5-mL filtered sample and a 10-mL spike. 3. The common complaint is that the end point is difficult to see. The following may help: (a) Dilute sample with more water to get a sharper end point. High conductivity, water hardness and alum can drag out the endpoint. Know the system prior to titrating. (b) Five to six drops of TB indicator (Remember to run the blank with the same number of drops). 4. For systems that ale quite close to neutral (k0.10 peg/ml) it is suggested that 20-mL filtered sample is used for the titration. 5. For most mills a 3-mL cationic spike is sufficient. Noi-nially TMP and CTMP furnishes will require a 10-mL polydadmac spike due to high anionic trash present. 6. A +0.2-mL titration difference between identical samples is considered the precision of the test. The test call give valuable information on the soluble colloidal charge of a paper mill system. It can give you an idea of cationic demand changes after treatment of headbox or thick stock samples with coagulants etc. the volume in the pulping and paper making systems, a large amount of time often stretching into days is required after treatment commences before a new equilibrium concentration of detrimental substances is achieved. OEFERENCES I. AUHORN W.J., MELZER J. Improved efficiency of wet end additives in closed wet end systems through elimination of detrimental siibstaiices. TAPPI Papermakers Conf. Proceed., Boston, 494(i C hances are that you recognize the names on this page. They are among the proven leaders in the industry. You ve come to trust their quality and performance. What you may not be aware of is that SANTXSALO is North America s manufacturer of these units. Should you need a new gear, need to rebuild an old gear box, or simply need service on any of your gearing, call us. When it comes to SANTASALO. Effect of M hite-water Contamination on Newspriq Properties. J. Pz~lpPa~~evSn. 11(4), , TRAFFORD J. Pitch investigation with PinusKadi- N/N bisiilphite and tliermomechanical pulps. Ap/)i/0:41(.7), , HEIMESZ P. Principles of Colloid and Surface chemist!-): Slarcel Dekker Iiic., New York, COOSEKSJ.W:S., LUNER P. Flocculation of microci-ystalline celliilose suspensions with cationic polymers: effect of agitation. TN H:59(2), 89-94, HAGEDORN R.A. The combination of highly charged polyelectrolytes with retention agents: retention iii the presence of interfering substances. T,W L70( 8), , ALLEN L.H. Deteriniliatioil of dispersed pitch Ixii.ticle coiiceiiti-atioiis. Pidp and Paper Research These two case histories demonstrate some of the benefits of the use of low molecular weight polymers for the control ofdetrimental substances in an integrated mills producing paper with mechanical pulp. At a very low dosage, excellent control of colloidal pitch can be achieved. At higher dosage, the control of detrimental substances greatly enhances the activity of retention aids. The benefits of this program including improved opacity and runnability have been demonstrated. Detrimental substances accumulate as a result of the recycle of white water. The control of these substances with low molecular weight cationic polymers is accomplished by changing the rate at which these materials are removed from the system with the papei web. Due to T408 Lph PULP & PAPER CANADA 94:12 (1993) 73