Converting Paper Mill Sludge to Papermaking Pulp and Filler Products

nd Filler Products

By John V. Maxham

After a sludge sample was re- New process technology boosts ceived, the consistency, ash content, Canadian Standard Freeness (CSF) amount of sludge solids recycled into raw and Bauer-McNett fiber-length dis- tribution (percent of OD solids re- paperma king materia 1s tained on 14-, 28-, 48- and 100-mesh screens) were measured. Most mills patented new process December 1992 and is now owned sent three to four sludge samples, technology can signifi- by Repap, a Canadian corporation. although some sent only one. cantly increase the total Prime Fiber, which is now de- Mill personnel collected sludge amount of sludge solids funct, held three US. patents on a samples that were representative of recycled into raw paper- process technology that converts the the sludge generated. Data were av- Amaking materials. The technology long-fiber fraction of paper mill eraged when separate samples were improves the drainage characteris- sludge into a papermaking pulp tested from a particular mill. Sludge tics of the fiber fines and clay frac- product. It successfully applied this consistency, which ranged from 16.1 tion remaining after long-fiber pulp technology at the Appleton mill percent to 48.6 percent and averaged is extracted and makes it suitable from April 1990 until August 1991. 31.6 percent, depended on the type for use as a filler in paperboard During that time, 1,922 metric tons of dewatering equipment used and products. It is estimated that 30 per- of AD market pulp was produced the type of sludge solids being de- cent of U.S. pulp and paper mills exclusively from paper mill sludge watered. Screw presses usually pro- have sludges in which the long-fiber and sold to Wisconsin Paper compa- duced the driest sludge cake, and yield is between 40 percent and 60 nies. Afterward, the company used belt presses produced a drier cake percent; the production of a filler predominantly post-consumer than either a vacuum filter or cen- product would increase the total white office wastepaper (PC-WOW) trifuge. Most of the mills that sent yield (long fiber and filler) to more as raw material to make a high- samples used belt presses. Sludges than 90 percent. brightness, low-dirt-count, 100 per- containing appreciable amounts of The technology was used in tan- cent post-consumer market pulp. biological solids and fiber fines were dem with a commercial long-fiber difficult to dewater, regardless of the recovery process in a recent study in Pulp and Paper Mill Sludge dewatering equipment used. which sludges from 33 North Amer- Characteristics and Quantities Ash content, which ranged from ican pulp and paper mills were eval- Prime Fiber initially received 2.3 percent to 55.7 percent and aver- uated. Based on that investigation, sludge samples from one Canadian aged 25.9 percent, depended on the researchers estimated that about 30 and 32 US. pulp and paper dsin- type of yrvduct the mill produced percent of the 4.2 million oven-dried terested in investigatingalternatives and the raw materials used to make (OD) metric tons of paper mill to their current disposal practices. The it. De-inking mill sludge typically sludge generated annually in the samples were tested to determine has a high ash content because its United States has sufficient pulp whether the long-fiber fraction could raw material (wastepaper)has a yield to be processed economically. be economically removed from the much higher ash content than virgin The study was conducted at the sludge and converted into a high- pulp. Coated paper mill sludge also former Prime Fiber Corp. market quality market pulp. Important has a high ash content because sig- pulp mill in Appleton, Wis., which considerationsincluded the amount nificant amounts of inorganic pig- had a nominal pulp production ca- of long fiber present as a percentage ments are added to the paper as pacity of 40 air-dried (AD) tons per of OD solids, the bleachability of the fillers or coatings. day. The mill was operated by long fiber, and the nature of the chem- The average CSF was 186, with a Prime Fiber from April 1990 until icals, dirt and debris in the sludge. range of 50 to 672. (CSF is a simple

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For more information, please complete the coupon below and mail or fax it to: Air & Waste Management Association, ‘3ne Gateway Ceriter, Third Floor, Pittsburgh, ...... PA 15222, phone: (412) 232-3444,...... fax: (412) 232-3450 Please provide information about A&WMA’s 88th Annual Meeting & Exhibition, June 18-23,1995, San Antonio, Texas Name Title Affiliation Address City State/Province Postal Code Country Phone Fax IW This skid of wet lap pulp was made from paper mill sludge at the Prime Fiber plant in Appleton, Wis. test to measure the rate of water tained appreciable amounts of large cent) had an estimated long-fiber drainage from a pulp sample. The woody debris and shives that usu- yield greater than 40 percent, and higher the CSF, the greater the ally were captured on the 14-mesh five (15 percent) had a long-fiber drainage rate.) CSF varied according screen. Fiber that passed through yield greater than 50 percent. Yield to the nature of the sludge solids, the 100-mesh screen was arbitrarily is a significant factor in determining particularly the organic or volatile considered to be fine. the economics of fiber recovery fraction. High percentages of long The Bauer-McNett test showed from paper mill sludge. Research fiber and large woody debris in a that as much as 75.5 percent and as shows that if the yield is much less sludge typically yielded a high CSF little as 12.9 percent of the OD than 40 percent, it is unlikely that value, while large amounts of biolog- volatile fraction in the raw sludge fiber recovery can be economically ical solids and gelatinous fiber fines samples was retained on screens up justified, even when the recovered resulted in low CSF values. If the to 100 mesh. On average, 43.8 per- fiber quality is good. Investigators long fiber was highly refined, its free- cent of the volatile fraction was re- therefore estimated that 30 percent ness also was low. (A CSF greater tained. As little as 5.6 percent and as of U.S. mills have sludge from than 600 is high, while a value less much as 59.3 percent of the total OD which fiber recovery may be eco- than 200 is considered low.) solids was retained on screens up to nomically attractive compared to The Bauer-McNett apparatus de- 100 mesh. On average, 33 percent, other disposal alternatives. termined the size of solids con- or about one-third, of the total OD A 1991 survey conducted by the tained in the sludge samples. Screen solids was retained. National Council for Air and Stream mesh sizes 14 (1.19-millimeter open- The total percent of OD solids re- Improvement, a research organiza- ing), 28 (0.595-millimeteropening), tained on Bauer-McNett screens up tion funded by the paper industry 48 (0.297-millimeteropening) and to 100 mesh can be assumed to be to deal with environmental issues, 100 (0.149-millimeter opening) were equal to the expected yield of a shows that between 55 and 60 kilo- used to fractionate the sludge solids. sludge processing facility recovering grams of dry waste solids are gener- Those retained on the screens were long-fiber pulp from paper mill ated per metric ton of paper or pa- predominantly long fiber, along sludge. (The actual fiber yield at the perboard produced. According to with small amounts of debris and Prime Fiber mill was estimated ac- the American Paper Institute, 72.9 ash. Sludge samples from integrated curately using this assumption.) million metric tons of paper and Kraft pulp mills sometimes con- Ten of the 33 mills (about 30 perboard were produced in 1990. Using

March/April 1995 37 an average of 57.5 kilograms of dry crease product quality and tonnage yield was too low to be economi- waste solids per ton as the average, while accepting poorer quality cally viable, the fiber could not be the pulp and paper industry pro- sludges. At the time, the company bleached to an acceptable brightness duced about 4.2 million OD metric was processing sludges from six or the sludge contained too much tons of waste solids that year. paper mills. dirt and debris for the screening and Table 1gives estimates of the The estimated amount of pulp cleaning system. Therefore, during total amounts of paper mill sludge, that could be produced from all six September 1991, Prime Fiber recoverable long fiber and filler gen- sources was 17.7 OD metric tons per switched from sludge to PC-WOW erated in the United States in 1990. day (19.7 AD metric tons per day), in its market pulp operations. The estimates are based on several considerably less than the 45 AD Table 3 gives the tonnage of paper assumptions: the average consis- metric tons per day hoped for. mill sludge processed by Prime Fiber tency of wet sludge is 31.6 percent, Though many other sludges were during the 17-month period. Sludge the ash content of the OD solids is examined, they proved unsuitable trucks were weighed before loading 25.9 percent and, on average, 5.8 for producing high-quality paper- to determine tonnage. At the same percent of the paper produced in the making pulp, because pulp recovery time, a technician would grab a sam- United States is converted to OD sludge solids, of which 33 percent could be converted to long fiber. In theory, about 1.4 million OD Estimated Quantities of Paper Mill Sludge Components metric tons of long fiber could have been recovered from the estimated Produced in the United States in 1990 4.2 million OD metric tons of paper (metric tons) mill sludge generated in 1990. The remaining 2.8 million OD metric Wet: 13,200,000 tons was fiber fines, ash and debris, Oven-dried: 4,200,000 of which 1.1 million was ash. Table 2 gives estimates on the Fiber fines, ash and debris: 2,800,000 amount and potential value of long fiber and filler that can be produced annually from paper mill sludges with long-fiber yields greater than 40 percent. The value of the recovered pulp was assumed to be between $400 and $600 per OD metric ton. Of the 33 mills whose sludges Estimated Fiber and Filler That Could Be Produced Annually were examined, eight (about 24 per- from U.S. Paper Mill Sludges with Long-fiber Yields cent) had yields greater than 36 per- Greater than 40 Percent cent and also had a potential fiber recovery greater than 7 metric OD Amount of fiber: 600,000 metric tons tons per day. Those mills would be considered possible candidates for I Potential.. value: $240 million to $360 million I fiber recovery. Of the 33 mills, four (about 12 Amount of filler: 500,000 metric tons percent) were considered excellent candidates for fiber recovery because the quality of recovered fiber was good and the sludge yielded a lot of recovered fiber. These abser- vations indicate that about 40 percent of the fiber and filler came from Prime Fiber Corp. Incoming Sludge mills where fiber recovery has excel- (April 1990 to August 1991) lent economic potential. Total wet tons: 13,512 The Prime Fiber Experience I Totrrl- - oven-dried- - - - . tons:. . 4.120I~ I From April 1990 to August 1991, Total ash tons: 516 Prime Fiber successfully produced market pulp exclusively from paper mill sludge. During that 17-month I Total yield, [OD. ,,DuID/OD sludqel:-. 42 percent I period, many changes in processing OD = ovendried. equipment and operations were Note: Weights are in metric tons. made to allow the company to in-

38 March/April 1995 t ’ I

ple so ash, consistency, volatile solids whenever sludges were processed ing the long fiber to a consistency and Bauer-McNett fiber-length distri- from Mills 1,2 and 6 (including up to 12 percent. bution could be measured. The landfilled sludge from Mill 1). After screening, the long-fiber Bauer-McNett test predicted that After the cleaning steps, long pulp entered a low-consistency (4 1,674 metric tons of fiber would be fiber was separated from the fiber percent) bleaching tank identical to recovered from 3,604 metric tons of fines and clay by a screening device. the defibering tank. Hypochlorite volatile solids entering the mill as Vibrating screens were used initially, was used as the bleaching agent. sludge. This prediction was about 2 but they had severe hydraulic ca- Steam was injected directly into the percent higher than the 1,645 metric pacity limitations and were replaced tank to raise the temperature to be- tons that were recovered. in June 1990 with sidehill screens tween 40 degrees and 50 degrees The six mills were charged a tip- that had a much higher hydraulic Celsius. After the DNT was in- ping fee that generally was set so capacity. Unfortunately, the sidehill stalled, hypochlorite bleaching was their costs were the same or slightly screens plugged easily and were re- performed in a medium-consis- less than landfilling. placed by December 1990 with a tency (10 percent to 12 percent) Prime Fiber personnel used an Black Clawson Double Nip Thick- bleaching tower. articulated front-end loader to han- ener (DNT). The DNT did an excel- After bleaching, the stock was di- dle the sludge after it was unloaded lent job of separating fiber fines and luted and bleach killer was added. on the tipping floor. The sludge was clay from the long fiber and thicken- The stock then was dewatered on a defibered (individual fibers were untangled from fiber bundles) at about 4 percent consistency in 22.7- cubic-meter, unbaffled stainless steel tanks. Agitation was provided by Prime Fiber Corp. Pulp Metric Tonnages 11.2-kilowatt agitators. Steam was (April 1990 to August 1991) injected directly into the tanks to raise the temperature to between 40 degrees and 50 degrees Celsius. In Total wet tons: 5,557 most cases, no pulping chemicals Total ovendried tons: 1,730 were added to the defibering tanks. One hour or less usually was suffi- I Total ash tons: 85 I cient to completely defiber a sludge. Total airdried tons: 1,922 Problems were encountered in defibering screw-pressed sludge, whose fibers were pressed tightly into ”bundles.” A deflaker was in- Pulp Properties stalled to help defiber this type of sludge. (When Prime Fiber switched Average Monthly Values to PC-WOW, the defibering tanks (April 1990 to August 1991) were replaced with a standard 3- OD-ton lobed hydrapulper.) CSF BRGT Dirt Ash After defibering, the furnish (a (ML) (“w (PPm) (“4 slurry of sludge fiber used to make Average 358 78.4 42.0 5.1 wet lap pulp) was pumped to a rec- Standard deviation 64 2.8 74.4 2.1 tangular grit tank that removed High 463 82.5 324.3 8.1 large, high-density materials. The stock then flowed to a 22.7-cubic- tow 260 71.5 5.5 23 meter consistency control tank, where it was diluted to about 1per- cent. Diluted stock initially was pumped to a bank of 0.15-meter-di- Pulp Properties ameter hydrocyclones (centrifugal Values of Monthly Composite Samples cleaners) with stock savers. These (March 1991 to August 1991) cleaners removed fine grit before the pulp went to screening. Eventually, Solids Ash CSF % Total Solids Retained the centrifugal cleaners were re- - (W (”/I (ML) on 100-Mesh Screen placed by a system that included Average 38.5 3.4 434 70.2 both coarse and fine pressure Standard deviation 0.7 0.6 26 3.2 screens and a forward, three-stage, 4.1 470 73.6 0.075-meter-diameter hydrocyclone. High 39.1 This equipment achieved dirt counts in the range of 5 ppm to 20 ppm

March/April 1995 39 I ‘* /1

belt press to a consistency in the Though the quality of the pulp The DNT had an approximately mid-20s. Dewatered stock was produced from paper mill sludge 100-mesh wire through which the stacked by hand on pallets and was not as high as virgin pulps, filtrate solids passed. Even so, only shipped to customers. In December market demand was still strong, 6.8 percent of the DNT filtrate solids 1990, the belt press was replaced by and prices were near virgin hard- was retained by a 325-mesh screen a wet lap pulp, which achieved a wood levels. on a Bauer-McNett apparatus. consistency in the high 30s. Though brightness was not mea- Ori@y, all effluents were clari- Converting Fiber Fines and Clay sured on these particular samples, fied in gravity sedimentation-type into Papermaking Filler nine other DNT samples collected units, whch pmved relatively ineffec- The main portion of the study fo- during the same time period had an tive and were replaced with a dis- cused on a new process technology, average brightness of 63.1, with a solved-air flotation (DAF) system.The developed with National Science low value of 56.2 and a high of 67.5. DAF efficiently removed waste solids Foundation grants, that converts the Brightness was affected by the pres- from effluent that was mycled to pdp- fiber fines and clay fraction of paper ence of fine ink particles. Attempts ing and consistency control. Waste mill sludge or PC-WOW into a pa- to brighten the fiber fines and clay DAF float solids were landfilled after permaking filler product after the fraction by bleaching the ink parti- being hckened in a rotary drum long fiber has been extracted. A rel- cles or removing the ink by flotation screen and dewatered by a screw press. atively uncontaminated stream of or adsorption onto a solid surface A total of 1,730 OD tons and 1,922 fiber fines and clay was contained in were unsuccessful. AD tons of pulp were produced exclu- the underflow or filtrate of the DNT. From March 1991 to August sively from paper mill sludge, whose Nineteen samples were collected 1991, samples of DNT filtrate solids overall pulp yield was 42 percent. between November 1991 and Febru- were collected when paper mill Pulp samples were taken from each ary 1992 (approximately one per sludge was used to make market truck load (about 18 metric tons of wet week), and the consistency, ash con- pulp. Many experiments were per- lap pulp) and measured for CSF, tent and Bauer-McNett retention formed on these samples to try to burst, brightness, dirt count and ash. were measured. The average consis- increase the brightness and decrease (Tables 4,5 and 6 show Prime Fiber’s tency was 0.088 percent, and the av- the specific resistance to filtration. pulp tonnage and quality data.) erage ash content was 33.5 percent. Specifically, the goal was to

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achieve a fiber fines and clay solids Thickening the DNT slurry and brightness greater than 80 and a spe- reacting it at ambient temperature cific resistance less than 1.0 Tm/kg [l with sufficient acid significantly terameter (Tm) = lo1*meters]. Values lowered pH, although a pH of 5.0 less than 1.O Tm/ kg are considered seemed adequate. Between 0.06 and desirable in wastewater treatment 0.12 grams of hydrogen sulfide per plant sludge dewatering operations. gram of OD DNT solids was needed Difficult-to-dewater sludges, such as to obtain a pH of 5.0. John K Maxham, a chemical engi- raw biological sludge, can have a spe Though acid treatment si@cantly neer, is an adjunct assistant professor in cific resistance as high as 100 Tm/kg improved the specific resistance of the the Paper Science Department at the and must be conditioned with ferric fiber fines and clay improving bright- Universityof Wisconsin’s Stevens chloride, lime, polymers or a combi- ness was much more difficult,particu- Point campus (StevensPoint, Wis.). nation of the three before dewatering. The DNT filtrate solid samples had a specific resistance between 1.75 and 8.78 Tm/kg, meaning they were mod- erately difficult to dewater in a vacuum filter apparatus. Because paper machine wires dewater the furnish by gravity and vacuum filtration, it was anticipated that adding raw fiber fines and clay mixtures to papermaking furnishes would cause drainage pmb- lems unless the specific resistance of the solids was decreased. For DNT filtrate solids obtained from paper mill sludge, brightness was sigruficantly increased by bleaching with hypochlorite. Even so, the results of bleaching experiments were somewhat disappointing, because fairly high dosages of hypochlorite often were required to achieve mod- est increases. A typical brightness gain was 1.0 to 1.5 points for every 1 percent of hypochlorite added (based on OD DNT solids) at dosages of less than 15 percent. Much greater success was achieved in improving the specific resistance. Experiments were performed in which the DNT filtrate solids were reacted with 15 percent hypochlorite at room temperature for 24 hours at a pH of 3,5,7,9 and 11. Reacting the fiber fines and clay at a low pH greatly improved the specific resistance, while reacting them at a high pH caused the specific resistance to increase significantly. Further experimentation showed that reacting the fiber fines and clay samples at low pH caused the solids to drain well, even if the pH was later increased. Also, it was unnecessary to add hypochlorite bleach to improve specific resistance. The ability to decrease the fiber fines’ specific resistance by lowering pH was a sigruficant and unexpected result. A process patent (No. 5,332,474) was issued on July 26,1994.

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