Oxygen detackification in secondary fiber recovery

Abstract: A method is disclosed for the detackification of stickies in the production of recycled paper products containing secondary fibers. Oxygen is used to reduce the tackiness of stickies in secondary pulps, and optionally is used with alkali and/or detackification agents for optimum stickies control.

Description: FIELD OF THE INVENTION This invention pertains to the recovery of secondary fibers from waste paper, and in particular to the detackification of tacky contaminants in waste paper furnish. BACKGROUND OF THE INVENTION Secondary fibers recovered from waste paper are converted into a wide variety of recycled paper products. Waste paper materials often contain varying amounts of troublesome sticky or tacky contaminants including adhesives, binders, coatings, certain types of inks, and the like. These tacky contaminants, usually referred to as "stickies", accumulate and cause operational problems in the various pieces of process equipment used to screen and clean the pulp, form paper sheet, and handle paper sheet products. The presence of these tacky contaminants or stickies also can affect adversely the quality of the final paper products. The growing utilization of secondary fibers in many types of paper products has made it necessary for paper manufacturers to process lower grades of waste paper. Such lower grade furnish is more heterogeneous and typically contains more stickies than a preferred higher quality waste paper. Economical manufacture of paper products using secondary fibers thus requires effective methods for treating secondary pulps to detackify and/or remove stickies (usually referred to as "detackification"). Known methods for removing or reducing the tackiness of stickies typically involve the addition of materials similar to those used to control pitch deposition in papermaking with virgin pulp. Such materials, defined herein as detackification agents, include surfactants, water-soluble polymers, and other ionic water-soluble compounds, as well as inorganic materials such as talc, diatomaceous earth, clays, and the like. These materials act to modify the surfaces of the dispersed particles of tacky compounds to remove the contact adhesive properties of the particles and thereby render them less troublesome and more easily removed. US-A-4,710,267 discloses the use of surface-active tertiary amines or quaternary ammonium compounds for reducing discoloration and tackiness in waste paper pulps. US-A- 4,886,575 discloses the application of polyvinyl alcohol having various molecular weights for detackification of waste paper pulps. P. C. Miller, in a paper entitled "Chemical Treatment Programs for Stickies Control" in TAPPI Proceedings , 1988 Pulping Conference, pp. 345-348, reviews the stickies problem and describes a number of approaches to detackification by chemical addition to secondary pulps. Oxygen treating or delignification of secondary pulp is known but generally has not been utilized to address the problem of stickies. US-A- 4,416,727 discloses a process for recovering and maintaining the brightness of fibers from wet-strength resin-coated waste paper furnish by pulping the furnish in an alkaline solution and contacting the pulp with oxygen to dissolve the polymeric wet-strength resins followed by recovery of the acceptable fibers in a washing step. An article by A. De Ruvo et al entitled "Upgrading of Pulp from Corrugated Containers by Oxygen Delignification" In Tappi Journal , June 1986, pp. 100-103 describes a process for upgrading recycled fibers by oxygen delignification to give the fiber a more virgin-like character by fiber swelling and softening resulting from lignin removal. The problem of stickies was identified but the removal of such stickies was not addressed. In an article entitled "Oxygen Bleaching of Secondary Fiber Grades" published In Tappi Journal , December 1988, pp. 168-174, L. D. Markham et al describe an oxygen bleaching process effective for removing most types of dirt and improving the brightness and bleachability of recycled pulps. The effect of the oxygen bleaching process on stickies was investigated and it was concluded that stickies were not well removed or dispersed in the oxygen stage. The need for improved and more economical methods for detackification of pulps in secondary fiber recovery led to the present invention as disclosed and claimed below. SUMMARY OF THE INVENTION The present invention is a method for treating a secondary pulp comprising cellulosic fibers and tacky contaminants, wherein the pulp is prepared from waste paper material. The method comprises contacting the pulp with an oxygen-containing gas in the absence of added alkali at conditions of temperature, oxygen partial pressure, and contact time sufficient to decrease the tackiness of the tacky contaminants, thereby reducing operating and product quality problems caused by the tacky contaminants during further processing of the cellulosic fibers to yield a recycled paper product. A second embodiment comprises the addition of one or more detackification agents to the pulp following contacting with an oxygen-containing gas, wherein the detackification agents are selected from the group consisting of ethoxylated surfractants, polyvinyl alcohol (PVOH), acetylenic surfractants, ionic surfractants, talc, clay, and mixtures thereof. A low-grade recycled paper product may thus be made from waste paper materials containing tacky contaminants by a method comprising (a) comminuting the waste paper with water to form a pulp comprising cellulosic fibers and tacky contaminants; (b) contacting the pulp with an oxygen-containing gas in the absence of added alkali at conditions of temperature, oxygen partial pressure, and contact time sufficient to decrease the tackiness of the tacky contaminants; and (c) processing the resulting pulp of cellulosic fibers from step (b) in one or more steps selected from the group consisting of pulping, screening, and papermaking to yield the low-grade recycled paper product. Optionally, one or more detackification agents can be added to the pulp during or following the contacting with oxygen. A high-grade recycled paper product may thus be made from waste paper materials containing tacky contaminants by a method comprising (a) comminuting the waste paper with water to form a pulp comprising cellulosic fibers and tacky contaminants; (b) contacting the pulp with an oxygen-containing gas in the absence of added alkali conditions of temperature, oxygen partial pressure, and contact time sufficient to decrease the tackiness of the tacky contaminants; and (c) processing the resulting pulp of cellulosic fibers from step (b) in one or more steps selected from the group consisting of pulping, screening, dispersing, washing, flotation, bleaching, and papermaking to yield the high-grade recycled paper product. Optionally, one or more detackification agents can be added to the pulp during or following the contacting with oxygen. DETAILED DESCRIPTION OF THE INVENTION The present invention is a method for the detackification of stickies in the processing of waste paper material to produce recycled paper products. Waste paper material includes both pre-consumer and post-consumer waste papers. Pre-consumer waste includes paper mill broke and sorted waste papers according to ISRI (Institute of Scrap Recycling Industries) scrap specifications; post-consumer waste includes office paper waste, curbside paper waste, news, magazines, and especially mixtures of these types of waste paper. A secondary pulp is a pulp made from such waste paper material. Stickies, also described as tacky contaminants, result from a number of precursor materials defined herein as including but not limited to pressure-sensitive adhesives, laser-printed inks, pitch, glue, hot melts, xerox inks, and latex. In existing plants for the recovery, cleaning, and bleaching of secondary fibers, different types of waste paper materials are segregated according to type (ledger, newspaper, cardboard, etc.) and are blended in specific ratios and pulped in a pulping apparatus such as a Hydrapulper. The pulp or furnish is subjected to one or more of the following steps depending upon the source of the furnish and the specific desired end product: screening, cleaning, deinking, bleaching, and washing. The treated pulp is then sent to the appropriate type of papermaking machine to form a final recycled paper product. Careful segregation of waste paper types yields a desirable high-quality furnish with reasonably consistent properties which is readily processed. Such higher-quality furnish contains varying amounts of stickies which must be detackified and removed to certain degrees depending upon the specific types of equipment used and the required properties of the final paper product. As the demand for secondary fiber increases, the supply of high-quality wastepaper is becoming insufficient and must be supplemented by lower-quality wastepaper materials. These lower-quality materials are generally more variable in properties than the more desirable higher-quality grades of wastepaper, and more importantly, typically contain higher amounts of stickies. As the use of stickies precursors (particularly contact adhesives) in paper products increases, and as the use of lower quality furnishes becomes necessary, the degree of operational problems caused by stickies in secondary fiber mills will become more severe. Thus improved and more economical methods of detackification will be needed to ensure the efficient production of paper products from secondary fibers. The present invention defines several new process options for the detackification of secondary pulps. Specific options can be chosen to yield the degree of detackification required for specific combinations of pulp processing equipment and specifications of final paper product. The degree of detackification selected is a balance between the cost of detackification and the cost of downtime to remove accumulated stickies from pulp processing equipment. It has been discovered in the present invention that contacting tacky adhesive materials, which form stickies, with an aqueous liquid in the presence of oxygen will reduce the tackiness of such materials to a significant degree. The contacting step can be carried out at ambient temperature, or at elevated temperatures as desired. Tackiness can be reduced further if a detackification agent is used in combination with the treatment using a heated aqueous liquid and oxygen. In a typical secondary fiber recovery operation, waste paper materials are selectively blended and pulped in water in a disintegration or comminution device such as a Hydrapulper. This raw secondary fiber pulp or furnish is then subjected to additional steps such as screening, cleaning, flotation, deinking, and bleaching in various sequences and combinations to yield a clean secondary fiber pulp which is converted into a selected paper product as is known in the art. The number of additional steps by which the raw pulp is treated will depend on the source of the waste paper material as well as the type and quality of the final paper product. Clean, well-segregated wastepaper will require fewer processing steps than lower quality wastepaper containing more dirt and high-lignin materials such as old corrugated containers, newsprint, and the like. High brightness or high quality paper products, defined as tissue, towel, printing paper, and writing paper, require higher degrees of screening and bleaching than products such as cardboard, tubestock, linerboard, and the like in which certain amounts of impurities are acceptable. As earlier discussed, stickies will be present in the waste paper in varying amounts. It is necessary to detackify and in most cases remove these stickies from the secondary fibers before proceeding through additional steps including the final papermaking step. The required degree of detackification and degree of removal of the detackified stickies will depend on the specific process steps and the type of final paper product. The present invention includes a number of embodiments which can be utilized to detackify stickies in any type of secondary fiber recovery sequence. In many instances, it will be desirable to detackify at the front end of the plant in or following the pulping step. In the first embodiment of the invention, the raw pulp is detackified by heating the pulp to between about 25 and 250°C, preferably between about 60 and 130°C, an contacting the pulp with oxygen at a partial pressure between about 34.5 and 3100.5 kPa (5 and 450 psig), preferably between about 137.8 and 689 kPa (20 and 100 psig), while mixing. Oxygen can be supplied by an oxygen-containing gas such as air, oxygen-enriched air, or high-purity oxygen containing between about 95 and 99.5 vol% oxygen. Typically this step will be continued for between about 0.1 and 120 minutes, preferably between about 5 and 40 minutes, to yield the required degree of detackification. While the mechanism of detackification by oxygen is not fully understood, it is believed that the stickies are hardened by chemical oxidation thus rendering the individual particles less prone to agglomerate and/or accumulate on the surfaces of plant equipment. This hardening also makes it less likely that particles will be retackified by shear in further pulp processing steps, in contrast with the use of detackification agents earlier described which only affect the surfaces of the sticky particles. In cases where a high degree of removal of completely detackified stickies is not necessary, such as in the production of corrugated medium, inner plys of cardboard, and boxboard, minimum further processing will be required before forming the final product. In most cases, however, the detackified stickies will be removed with other impurities in screening, centrifuging, flotation, and other physical contaminant removal steps. In an other embodiment one or more detackification agents are added to the pulp following the treatment with oxygen to give further reduction of stickies. This embodiment may be appropriate with certain types of stickies and furnishes in which a high degree of detackification is needed but fiber damage must be minimized and pulp yield maximized. In such cases, the use of detackification agents complements the use of oxygen and renders the detackification process more flexible. Detackification agents can be selected from the group consisting of ethoxylated surfactants, polyvinyl alcohol (PVOH), acetylenic surfactants, ionic surfactants, talc, clay and mixtures of these agents, and are used at doses of between about 1 and 10,000 parts per million (ppm) by weight based on dry pulp. The pulp is maintained at a temperature between about 20 and 150°C, preferably between about 20 and 80°C, when these agents are added. In another embodiment of the invention one or more detackification agents are added to the pulp following the treatment with oxygen to give further reduction of stickies. This embodiment may be appropriate with certain types of stickies and furnishes in which a high degree of detackification is needed but fiber damage must b minimized and pulp yield maximized. In such a case, the use of detackification agents complements the use of oxygen and renders the detackification process more flexible. Detackification agents, doses, and temperatures are selected as described above. A low-grade recycled paper product may be made by the process of the present invention, wherein the term low-grade paper product includes but is not limited to low-grade corrugated media, inner plies of cardboard, molded products such as trays and cartons, and tubestock. These low-grade papers have the appropriate strengths for specific uses, but appearance is not important so that high brightness and optical homogeneity are not necessary. A high-grade recycled paper product may also be made by the process of the present invention, wherein the term high-grade paper product includes but is not limited to newsprint, towel, tissue, printing paper, and writing paper. In these products, appearance is critical and high brightness and optical homogeneity are required. A series of laboratory experiments was carried out to quantify and compare various treatment steps for the reduction in tackiness of two typical adhesive materials which are precursors to typical stickies found in secondary pulps. These experiments are summarized in the following Examples. EXAMPLE 1 Paper coupons were coated with vinyl acetate-acrylate pressure-sensitive adhesive. One group of coupons was immersed in tap water at ambient conditions for 10 minutes. A second group of coupons was immersed for 10 minutes at ambient conditions in water containing dissolved NaOH at 5.0 g/l. A third group of coupons was immersed at ambient conditions for 10 minutes in water containing 5 wt% NaOH and 2.0 wt% of sodium sulfate to simulate fully oxidized white liquor. Selected coupons were placed on a tray rack in a reactor pressurized to 620.1 kPa (90 psig) with either water-saturated oxygen or water-saturated nitrogen. The coupons were held in the reactor for 45 min at either 85°C or 130°C, removed therefrom, and air-dried. Two of the original coated coupons were immersed for 45 minutes at 80°C in virgin pulp filtrate, which is the liquor contained in kraft pulp after washing prior to oxygen delignification. This liquor typically contains about 1.0 wt% lignin an wood cellulose components and has a pH of about 9. similarly in virgin pulp filtrate but were not treated with detackification agents and. All coupons were then rinsed with water and air-dried. These treated coupons and one of the original coated coupons were then tested for tackiness using a Polykem Series 400 probe tack tester. Each coupon was placed tacky side down on the surface of a flat annular ring and a standard flat weight of 10 grams was placed atop the coupon, pressing it against the ring. A piston with a flat face was raised through the center of the annular ring and contacted with the tacky surface of the coupon. The piston was then lowered and the force necessary to separate the piston surface from the tacky coupon was measured and the tackiness in grams/cm² was calculated. This was repeated 10 times and the average determined for each coupon. The results of these tests are summarized in Table 1. The remaining coated coupons were used as controls or were treated at temperatures of 30-85°C and 45 minutes contact time with detackification agents at 40 ppm by weight dissolved in virgin pulp filtrate. Tackiness was measured for each coupon using the Polyskem probe tack tester as above. The results of these tests are summarized in Table 2. TACKINESS OF TREATED AND UNTREATED VINYL ACETATE-ACRYLATE COATED PAPER COUPONS Sample Detackification Treatment Tackiness, g/cm² Detackifier Temperature, °C Original 2 - - 523 Control 3 - 30 400 Control 4 - 60 335 Control 5 - 85 389 11 PVOH 540 30 228 12 PVOH 540 60 153 13 PVOH 540 85 203 14 Surfynol 485 60 180 15 Surfynol 485 85 204 16 Nonylphenol Ethoxylate 60 166 17 Nonylphenol Ethoxylate 85 199 18 Dodecylphenol Ethoxylate 60 196 19 Dodecylphenol Ethoxylate 85 202 The results of these tests confirm that these detackification agents alone reduce tackiness as expected. It appears that 60°C is the optimum temperature for detackification of vinyl acetate-acrylate with these agents. The use of oxygen alone at higher temperatures is surprisingly more effective, and when combined with detackification agents is even more effective at reducing tackiness. EXAMPLE 2 Paper coupons were coated with styrene-butadiene rubber pressure-sensitive adhesive, and detackification tests were performed on these coupons using the procedures of Example 1. The results of these tests are summarized in Tables 3 and 4 and lead to the same conclusions as in Example 1. TACKINESS OF TREATED AND UNTREATED STYRENE-BUTADIENE RUBBER COATED PAPER COUPONS Sample Detackification Treatment Tackiness, g/cm² Detackifier Temperature, °C Original 1 - - 505 Control 2 - 30 405 Control 3 - 60 314 6 PVOH 540 30 242 7 PVOH 540 60 205 8 Surfynol 485 60 185 9 Nonylphenol Ethoxylate 60 204 10 Dodecylphenol Ethoxylate 60 225 EXAMPLE 3 Additional tests at various conditions were carried out according to the initial treatment steps of Example 1 using test coupons coated with vinyl acetate-acrylate. The tests utilized oxygen, nitrogen, and air for reactor pressurization. The results as given in Table 5 show that oxygen alone is an effective detackifier. TACKINESS OF TREATED VINYL ACETATE-ACRYLATE COATED PAPER COUPONS Gas Alkali T, °C P, kPa (Psig) Retention Time, min. Tackiness, g/cm² --Untreated-- - - - 521 Air - 25 101.3 (14.7) 3 498 O₂ - 25 101.3 (14.7) 3 295 Air - 85 620.1 (90) 45 381 N₂ NaOH 85 620.1 (90) 45 190 O₂ - 85 620.1 (90) 45 156 O₂ NaOH 85 620.1 (90) 45 114 Air - 356 Air NaOH 130 620.1 (90) 45 125 N₂ - 130 620.1 (90) 45 356 N₂ NaOH 130 620.1 (90) 45 160 O₂ - 130 620.1 (90) 45 124 O₂ NaOH 130 620.1 (90) 45 113 EXAMPLE 4 Laser printed ledger paper was pulped in a laboratory reactor with water for 15 minutes and a portion of the pulp was made into a handsheet. The sheet contained large numbers of small specks of laser printed ink and had a measured brightness (Tappi T 217 wd-71) of 77.5. The specks were too small to physically characterize as sticky or tacky, but laser printed ink is known to deposit on pulp processing equipment in secondary fiber recovery. Two portions of the remaining pulp were treated individually in a laboratory reactor under either a nitrogen (as a control) or an oxygen atmosphere at 620.1 kPa (90 psig) and 130°C for 60 minutes. The pulp was mixed at 800 RPM every 10 minutes. Pulp samples were removed from the reactor and each was screened through a six-cut Sommerville vibrating screen with slots 0.15 mm wide and 45 mm long. Handsheets were prepared from each screened pulp sample, qualitatively observed, and tested for brightness. Visual observations of the handsheets showed that the control pulp contacted with nitrogen yielded a darker sheet than the untreated pulp, indicating that the laser ink was further dispersed and the fibers darkened. The oxygen-treated pulps yielded sheets which were almost completely cleaned of ink particles, with an increased brightness. This indicated that the laser-printed ink particles were hardened and agglomerated so that removal by screening was readily accomplished. Ink removal can be accomplished by oxygen without alkali. It is expected that this method also will remove any type of xerox or electrostatic ink from secondary pulps. A summary of the data from these tests is given in Table 6. Removal of Laser Printed Ink Gas Used NaOH Source NaOH Dose, w% on dry pulp Brightness Untreated -- -- 77.5 Nitrogen -- -- 73.9 Oxygen -- -- 82.7 Application of the detackification methods described in Examples 1-4 illustrate the embodiments of the present invention. The quantitative tests described in Examples 1 and 2 are useful for determining and comparing the effectiveness of detackification methods for various adhesive materials which are the precursors of stickies. Because it is not possible to quantify the actual reduction in tackiness of individual particles or stickies dispersed in pulp on a laboratory scale, the qualitative tests described in Examples 3 and 4 can be useful in estimating the detackification characteristics of new sources of waste paper materials prior to processing for secondary fiber recovery, and also to monitor for quality control purposes the characteristics of waste paper regularly obtained from a specific source. Thus the embodiments of the present invention are new and useful methods for improving the operation of secondary fiber recovery processes by reducing the problems caused by stickies deposition in the equipment used in these processes. The use of oxygen to detackify an aqueous pulp suspension is a simple, effective, and economical method and in many cases may be the only detackification step needed. It can be more economical in some cases than the use of detackification agents alone, especially if oxygen is already available at a given secondary fiber plant. The use of oxygen can complement the use of detackification agents, and this combination is potentially useful and economical when a high degree of detackification is necessary along with a high pulp recovery and minimum fiber damage.

Claims: A method for treating a secondary pulp comprising cellulosic fibers and tacky contaminants, wherein said pulp is prepared from waste paper material, said method comprising contacting said pulp with an oxygen-containing gas in the absence of added alkali at conditions of temperature, oxygen partial pressure, and contact time sufficient to decrease the tackiness of said tacky contaminants, thereby reducing operating and product quality problems caused by said tacky contaminants during further processing of said cellulosic fibers to yield a recycled paper product. The method of Claim 1 wherein said contacting is carried out at a temperature between about 25 and 250°C. The method of Claim 2 wherein said contacting is carried out at a temperature between about 60 and 130°C. The method of Claim 1 wherein said contacting is carried out at an oxygen partial pressure between about 34.5 and 3100.5 kPa (5 and 450 psig). The method of Claim 1 wherein said contacting is carried out a contact time of between about 0.1 and 120 minutes. The method of Claim 1 further comprising the addition of one or more detackification agents to said pulp following said contacting with oxygen-containing gas. The method of Claim 6 wherein said one or more detackification agents are selected from the group consisting of ethoxylated surfactants, polyvinyl alcohol (PVOH), acetylenic surfactants, ionic surfactants, talc, clay and mixtures thereof. The method of Claim 6 wherein said one or more detackification agents are added to said pulp in a total amount ranging between about 1 and 10,000 ppm by weight based on pulp.

Inventors: HRISTOFAS KONSTANTINOS, MAGNOTTA VINCENT LOUIS, NADDEO RONALD CHARLES, HRISTOFAS, KONSTANTINOS, MAGNOTTA, VINCENT LOUIS, NADDEO, RONALD CHARLES
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1992
Main classification: D21C5
IPC codes: D21H17, D21C5, D21C9