BACKGROUND OF THE INVENTION In the manufacture of paper, a cellulosic pulp is first created and used to make paper. In some manufacturing processes the pulp and paper are made at the same manufacturing location. When pulp and paper are made at the same manufacturing location it is possible to make the pulp and easily transport it in its wet state to the papermaking plant. In other manufacturing processes the pulp and paper are made at separate facilities and thus it is necessary to ship pulp from the pulp manufacturing location to the paper manufacturing location. Due to the prohibitive cost of shipping pulp in its normal wet state, processes and equipment have been developed to dewater pulp so that it can be transported in wet lap or dry lap form to paper making facilities wherever it is needed.

There are only two major types of pulp drying machines, which differ in the manner in which the pulp sheet is formed. These pulp drying machines are known as "fourdrinier"and"cylinder"machines. The choice of what type of pulp drying machine to use is dependent upon the type of pulp being dried and the desired final consistency and people of ordinary skill in the art know how to do this.

In a fourdrinier machine, the pulp (also known as a "stock slurry"at from about 1% to about 2% consistency) is deposited from a headbox onto a continuous, moving open mesh fabric. The water in the slurry drains through the fabric thus forming a pulp mat. After the initial free drainage through the fabric or wire, the mat is further dewatered as it is carried on the wire by the application of progressively increasing vacuum. The vacuum is applied to the underside of the mat by a series of elements known as vacuum boxes. Vacuum may also be applied at the couch roll, just prior to the removal of the mat from the forming fabric. Nominal mat consistency at this point is about 16% to about 20%. The pulp mat then enters the press section of the machine, which typically consists of from two to four press nips, where further water is removed by mechanical expression. The nominal mat consistency after pressing is from about 40% to about 45%. After the press section, further water is removed by evaporative means, typically by hot air impingement. The final consistency of the sheet is typically in the range of from about 81% to about 86% (oven-dry basis) or from about 90% to about 95% (air-dry basis).

In a cylinder machine, the stock slurry is contained in a vat and a rotating, fabric-covered cylinder is used in forming the mat. The stock in the vat is picked up onto the cylinder and drainage of water occurs through the fabric/screen to form the mat. The mat is further vacuum dewatered on the cylinder, whereupon it is transferred to a press section and dryer section as already described for the fourdrinier machine.

It is desired to maximize dewatering of the pulp because of the need to remove as much moisture as possible to reduce the weight of the pulp to make transporting the dried pulp as cheap as possible. It is also advantageous to remove as much water as possible in the earlier stages of the process as it becomes progressively more expensive to remove water in going from the forming to the vacuum to the press to the dryer sections of the machine. For example, every 1W increase in consistency after the press section can be expected to yield a 4% decrease in energy costs. Alternatively, the improvement in press solids can be used to increase production by approximately a similar 4% amount.

Dewatering can be maximized by working to achieve optimal performance of the mechanical water removal sections (vacuum, press, and dryer). In order to maximize dewatering, the stock temperature is kept as high as possible, typically as high as from about 150°F to about 160°F (about 65°C to about 71°C) to enhance water removal by lowering the water viscosity. Also, steam boxes can be used to increase the temperature of the mat prior to the press section. A 10°C increase in the web temperature can be expected to yield a 1% improvement in press solids. The pulp pH is kept low, from about 4 to about 5, so that the fibers are less swollen and drain easier.

It is not standard procedure to use polymeric chemical drainage aids to enhance vacuum dewatering due to the fact that most polymeric dewatering aids are not effective.

U. S. Patent No. 4,062,721, USE OF SURFACTANT TO INCREASE WATER REMOVAL FROM FIBROUS WEB, describes and claims a method for removing water from a wet fibrous sheet, by adding a"surfactant foaming agent to the aqueous slurry'"comprising mineral wool and a binder.

The"aqueous slurry"in this patent must include mineral wool and the surfactant foaming agent is recommended to be a"non-ionic surfactant". The patent does allow for the use of"cationic and anionic surfactants" (column 5, lines 43-46), however, the data shown (Summary of Experiments Table in columns 3 and 4) in this patent indicated that anionic surfactant Triton @W-30 did not perform better (% solids of 35.5) than the control situation where no surfactant was added (% solids of 37.5).

It would be desirable to find an additive that increased the effectiveness of vacuum dewatering of cellulosic pulp in a pulp dryer.

SUMMARY OF THE INVENTION This invention is a method to enhance the vacuum dewatering of cellulosic pulp comprising adding between about 0.05 lb/T and about 10 lb/T of an anionic surfactant to said cellulosic pulp after said cellulosic pulp is washed and before said cellulosic pulp is vacuum dewatered.

DETAILED DESCRIPTION OF THE INVENTION For purposes of this patent application, the following terms have the indicated meanings: "alkyl"means a fully saturated hydrocarbon radical of from about 1 to about 40 carbon atoms, which may be linear or branched.

"aryl"means an aromatic monocyclic or multicyclic ring system radical of about 6 to about 20 carbon atoms.

"AcAm/DMAEM-MCQ"means copolymers from acrylamide and dimethylaminoethylmethacrylate methyl chloride quaternary.

"AcAm/DMAEA-MCQ"means copolymers from acrylamide and dimethylaminoethylacrylate methyl chloride quaternary.

"alkylaryl"means an aryl moiety with at least one alkyl substituent.

"alkanolamine"means compounds of the formula H2N-Rf- OH, where Rf is alkyl.

"alum"means aluminum sulfate.

"cellulosic pulp"is one made from wood or plant materials.

"consistency"is : dry weight of fiber total weight of fiber plus water "cycloalkyl"means a fully saturated hydrocarbon radical of from about 3 to about 40 carbon atoms, which must be cyclic.

"dry lap"refers to pulp after it has been vacuum dried and heated.

"EDC/NH3 polymers"means polymers made by reacting ethylene dichloride and ammonia.

"lb/T"means pounds of active ingredient per Ton (2000 pounds) of dry pulp.

"lb product/T"means pounds of total product (not just active ingredient) per Ton (2000 pounds) of dry pulp.

"PAC"means polyaluminum chloride, and other Aluminum-based coagulants.

"paraffin sulfonates"are prepared by sulfoxidation of paraffin hydrocarbons.

"petroleum sulfonates"are prepared by reaction of selected petroleum refining fractions with sulfuric acid or oleum.

"polyacrylic acid"means polymers from acrylic acid or hydrolysis of polyacrylamide.

"polyacrylate"means salt neutralized form of polyacrylic acid (salt can be sodium, potassium, etc).

"polyamines"means polymers containing amine functionality, such as dimethylamine-epichlorohydrin polymers.

"polyamidoamines"means polymers containing amine and amide functionality such as condensation polymers of diethylene triamine and adipic acid.

"poly polymers from diallyldimethyl ammonium chloride.

"poly (DMAEM means a homopolymer of dimethylaminoethylmethacrylate methyl chloride quaternary.

"poly (DMAEA-MCQ)" means a homopolymer of dimethylaminoethylacrylate methyl chloride quaternary.

"polyethyleneimine"means polymers synthesized from ethyleneimine.

"polyvinylamine"means polymers made from the polymerization of N-vinyl formamide which polymers are then hydrolyzed. This also includes copolymers of vinylamine with monomers such as vinylformamide, vinyl acetate, vinyl alcohol and acrylamide.

"sodium dodecyl sulfate"is the same as sodium lauryl sulfate.

"Stepan"is Stepan Company, 22 W. Frontage Road, Northfield, IL 60093 (847) 446-7500.

"wet lap"means pulp before it has been heated.

This invention is a method to enhance the vacuum dewatering of cellulosic pulp comprising adding between about 0.05 lb/T and about 10 lb/T of an anionic surfactant to said cellulosic pulp after said cellulosic pulp is washed and before said cellulosic pulp is vacuum dewatered.

Anionic surfactants suitable for use in the instant claimed invention include anionic surfactants selected from the group consisting of Alcohol Sulfates, Alcohol Alkoxy Sulfates, Sulfonates, Dialkyl Sulfosuccinates, and Sulfosuccinic Acid Ester with Ethoxylated Alcohols and mixtures thereof.

"Alcohol Sulfates"means compounds of the formula ROSO3M, where R is Cg-Cis alkyl. The alcohol sulfates exist either in the salt form, where M is selected from the group consisting of sodium, potassium, magnesium, ammonium or alkanolamine, or in the acid form where M is H. Examples of alcohol sulfates suitable for use in the instant claimed invention include sodium dodecyl sulfate, tetradecyl sulfate, ammonium lauryl sulfate, magnesium lauryl sulfate, cetyl sulfate, octyl sulfate, nonyl sulfate, decyl sulfate and 4-undeconal, 7-ethyl-2-methyl- sulfate, sodium salt, a/k/a Niaproof Anionic Surfactant 4 (available from Niacet Corporation, 400 47th Street, Niagra Falls, New York 14304,716-285-1474.) "Alcohol Alkoxy Sulfates"means compounds of the formula RO (RgCH2CH20) xS03M where R and M are as defined above and where Rg is-CH2 or non-existent, and where x is an integer from 1 to 50. Alcohol alkoxy sulfates exist in the acid or salt form. One alcohol alkoxy sulfate suitable for use in the instant claimed invention is ammonium lauryl ether sulfate.

"Sulfonates"means compounds of the formula RaSO3M where Ra is a Gs-Cis alkyl group, a C3-C12 cycloalkyl group or a Cs-Cis aryl group or a Ca-C18 alkylaryl group.

Examples of sulfonates suitable for use in the instant claimed invention include dodecyl sulfonate, tetradecyl sulfonate, alkylbenzenesulfonic acids (ABSA) and salts of alkylbenzenesulfonic acids, alkylbenzenesulfonates (ABS), linear alkylbenzenesulfonates, paraffin sulfonates, petroleum sulfonates and alpha olefin sulfonates.

"Dialkyl Sulfosuccinates"means compounds of the formula RbOOCCH2CH (SO3M) COORb, where Rb is selected from the group consisting of amyl, octyl (2-ethyl hexyl), isobutyl, tridecyl, and lauryl. M is as defined above.

Examples of Dialkyl Sulfosuccinates suitable for use in the instant claimed invention include dioctylsulfosuccinate.

"Sulfosuccinic Acid Ester with Ethoxylated Alcohols" means compounds of the formula: Rc (OCH2 CH2) nOOCH (SO3M) CH2COOM where Rc is a Cg-Cis alcohol, n is an integer from 1 to 10, and M is as defined above. An example of a Sulfosuccinic Acid Ester with Ethoxylated Alcohol suitable for use in the instant claimed invention is laureth sulfosuccinate (a/k/a as Schercopol LPS, available from Scher Chemicals, Inc. Industrial West., Clifton, NJ 07012-0997, (201) 471-1300).

All of these anionic surfactants are available commercially or can be readily manufactured using techniques known in the art.

The preferred surfactant is sodium dodecyl sulfate (a/k/a as sodium lauryl sulfate), available from Stepan in a 30% actives solution.

The amount of anionic surfactant is optimized based upon the type of pulp. Typical dosing of anionic surfactant is in the range of from about 0.05 lb/T to about 10 lb/T, preferably in the range of from about 0. 11b/T to about 4 lb/T, most preferably in the range of from about 0.2 lb/T to about 2 lb/T, and most highly preferably in the range of about 0.25 lb/T and about 0.75 lb/T. It is of course possible to use more anionic surfactant than that amount specified here, but it is understood that using more anionic surfactant will be uneconomical and will not lead to an increase in performance.

The surfactant is added to the pulp, after the pulp has been washed and before the pulp is vacuum dried.

Preferably, the anionic surfactant is added to the pulp just prior to mat formation. A very suitable point of addition for the anionic surfactant is to add the surfactant just prior to the headbox on a fourdrinier pulp dryer. The anionic surfactant can be added in combination with other materials to enhance the dewatering of the pulp. These other materials can include defoamers and coagulants.

Defoamers useful in enhancing the dewatering of pulp when used in combination with anionic surfactants include C8-C48 alcohols in water with an emulsifier, Cg to C48 alkoxylated alcohols, such as ethoxylated propoxylated alcohols, silicones, wax (in ppm levels only), silica and ethylene bis stearamide (particulate suspended in oil) compounds, and blend (s) of a triglyceride ester and a polyethylene glycol ester. The preferred defoamer is a 10% actives blend of C16 and C18 alcohols. All of these defoamers are available commercially from Nalco or other companies, or can be readily synthesized using techniques known in the art.

The amount of defoamer is optimized based upon the type of pulp and the specific anionic surfactant being used.

Typical dosing of C8-C48 alcohol defoamer is in the range of from about 0.5 lb product/T to about 10 lb product/T, preferably in the range of from about 0.75 lb product/T to about 7 lb product/T, most preferably in the range of from about 1 lb product to about 3 lb product/T and most highly preferably about 2 lb product/T. The dosage of other types of defoamers would typically be less.

When defoamer is added with anionic surfactant, the defoamer can be added prior to addition of the anionic surfactant, after the addition of the anionic surfactant or simultaneously with the addition of the anionic surfactant. Preferably, the defoamer is added simultaneously with the anionic surfactant.

Coagulants useful in enhancing the dewatering of pulp when used in combination with anionic surfactants include those selected from the group consisting of polyamines, polyamidoamines, polyethyleneimine, poly (DADMAC), EDC/NH3 polymers, AcAm/DMAEM-MCQ, AcAm/DMAEA-MCQ, poly (DMAEM-MCQ), poly (DMAEA MCQ), polyacrylic acid or polyacrylate, copolymers of DADMAC and acrylic acid, copolymers of DADMAC and AcAm, alum, PAC, polyvinylamine, and copolymers of vinylamine with vinylformamide, vinyl acetate, vinyl alcohol, and acrylamide.

All of these coagulants are available commercially or can be readily synthesized using techniques known in the art. Typical dosing of organic coagulants is in the range of from about 0.1 lb product/T to about 20 lb product/T, preferably in the range of from about 0.2 lb product/T to about 10 lb product/T, and most preferably in the range of from about 0.5 lb product/T to about 6 lb product/T.

When coagulant is added with anionic surfactant, the coagulant can be added prior to addition of the anionic surfactant, after the addition of the anionic surfactant or simultaneously with the addition of the anionic surfactant. Preferably, the coagulant is added before the anionic surfactant and before the defoamer, if a defoamer is added. Preferably the anionic surfactant is added with a defoamer.

The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill how to practice and make use of the invention. These examples are not intended to limit the invention or its protection in any way.

EXAMPLES Background Information for Laboratory Testing on Vacuum Drainage Tester (VDT) The following operating conditions are in effect in setup for the examples of laboratory testing: VDT is a Vacuum Drainage Tester that permits mixing of pulp in the vacuum drainage tester. It is available from Nalco ; Filter paper type is Ahlstrom 1278 (available commercially from Ahlstrom Filtration, Inc., 122 West Butler Street, Mount Holly Springs, PA 17065 (800) 326- 1888); Stirrer Speed is 700 rpm; Applied Vacuum is 15 in. Hg; Pulp Volume is 500 ml; Pulp Consistency is 1. 66% ; Pulp temperature is about 150°F (65°C) ; and The standard time vacuum is applied following air breakthrough in the pulp pad is about 1 minute.

The process for testing the anionic surfactant is as follows: a) add the pulp to the vacuum drainage tester; b) add the anionic surfactant to the pulp in the vacuum drainage tester; c) mix for about 20 seconds; and d) apply vacuum and allow drainage to begin.

Laboratory Example 1 The pulp tested is a Northern hardwood kraft market pulp. Surfactant Type Dose Consistency (Ib/T) (%) None 0 21. 46 Sodium dodecyl sulfate Anionic 0.5 24.71 1 25. 2 27. 25 4 28. 61 Alpha-olefin sulfonate Anionic 0.5 24. 11 1 25. 51 2 27. 08 4 28. 52 Dodecylbenzene sulfonic acid Anionic 0.5 24.16 1 25. 56 2 27. 19 4 28. 65 An increase in consistency shows a POSITIVE effect upon dewatering, because higher consistency means that the pulp dewatered more. Another way of saying this is that the dewatering of the pulp was enhanced by the presence of the anionic surfactant.

Laboratory Example 2 Test conditions are the same as Laboratory Example 1 except that stirrer speed is GOO rpm; pulp consistency is 1.68%; pulp temperature is about 60 °C ; the pulp tested is a Northern hardwood kraft market pulp Surfactant Type Dose Consistency (Ib/T) (%) None 0 21. 06 Sodium dodecyl sulfate Anionic 1 22. 21 Ammonium lauryl sulfate Anionic 1 23.19 Ammonium lauryl ether sulfate Anionic 1 22.80 (4 moles EO) Ammonium lauryl ether sulfate Anionic 1 22. 96 (12moles EO) Tetradecyl sodium sulfate Anionic 1 22. 49 Laboratory Example 3 Test conditions are the same as Laboratory Example 1 except that stirrer speed is 1200 rpm; vacuum is 20 in.

Hg; pulp volume is 900 ml; pulp consistency is 1.83%; temperature is from about 150°F to about 160 °F (65°C to about 71°C) ; vacuum applied for 10 seconds after break. The pulp tested is a Southern hardwood kraft market pulp. Surfactant Type Dose Consistency (lb/T) (%) None 0 20. 99 Sodium dodecyl sulfate Anionic 0.25 21.83 0. 5 22. 53 1 23. 44 2 23. 97 4 24. 03 Sodium decyl sulfate Anionic 0.25 21.22 0.5 21.64 1 22. 20 4 23.71 Alpha-olefin sulfonate Anionic 0. 25 21. 76 0. 5 22. 26 1 22. 96 4 24.16 Laboratory Example 4 Test conditions are as follows: instrument is a Dynamic Drainage Analyzer (DDA), available from AB Akribi Kemikonsulter, Högalidgaton 26, S-856 31 Sundsvall, Sweden, telephone number 46 60 66 49 50. A DDA is similar in operation to a VDT except automated; stirrer speed is 1000 rpm; applied vacuum is 0.2 bar; pulp volume is 600 ml; pulp consistency is 1.32%; temperature is 70°F (21°C) ; vacuum applied for 1 minute after break; screen size is 0.25 mm. The pulp tested is a Northern hardwood kraft market pulp. Surfactant Type Dose Consistency (Ib/T) (%) None 0 19. 98 0. 30 Sodium dodecyl sulfate Anionic 2 22. 30 Sodium laureth sulfate Anionic 2 21. 39 Sodium octyl sulfate Anionic 2 20. 28 Dioctyl sulfosuccinate sodium salt Anionic 21. 67 Laboratory Example 5 Test conditions are the same as in Laboratory Example 1 except for the following: pulp consistency is 1.6% ; temperature is from about 150°F to about 160 °F (65°C to about 71°C). The pulp tested is a Northern hardwood kraft market pulp. Surfactant Type Dose Consistency (Ib/T) (%) None 0 22. 30 Sodium dodecyl sulfate Anionic 1 25. 55 3 27. 31 5 28. 25 Laureth sulfosuccinate Anionic 1 22. 99 3 24. 30 5 25. 67 2-ethylhexyl sulfate Anionic 1 22. 37 3 23.14 5 24. 01 Tetradecyl sodium sulfate Anionic 1 26. 34 3 27. 74 5 28. 90 Dioctyl sulfosuccinate Anionic 1 25. 34 sodium salt 3 27.28 5 27. 82 Laboratory Example 6 Test conditions are the same as in Laboratory Example 1 except for the following: stirrer speed is 800 rpm; pulp consistency is 1.6% ; temperature is about 155°F (68.3°C); pH is 3.9. The pulp is a Northern hardwood kraft market pulp. Surfactant Type Dose Consistency (Ib/T) (%) None 0 21. 17 + 0. 02 Triton W-30 Anionic 1 22. 66 3 24. 82 5 25. 91 10 27. 84 Sodium dodecyl sulfate Anionic 1 24. 61 3 26. 98 5 28. 62 Ethoxylated dodecyl sulfate Anionic 1 22. 62 (50 mole EO) 3 25. 60 5 27. 63 These laboratory examples show that the use of anionic surfactant consistently enhances the vacuum dewatering of different types of cellulosic pulp.

Plant Trial Example 1 1St Trial at Roadrunner Pulp Mill in Irontown, MI A plant trial is held at the Roadrunner Pulp Mill in Irontown, Michigan. The mill produces approximately 1200 Tons (2000 pounds)/day of elemental chlorine free ("ECF") bleached northern hardwood kraft market pulp.

The pulp is bleached to a 92.5 ISO brightness level. It is composed primarily of maple, birch, beech, and aspen fiber types.

The pulp dryer machine used to manufacture this pulp is a fourdrinier type manufactured by the Black Clawson Company. The fourdrinier machine is equipped with eight functioning vacuum flat boxes, a suction couch roll, one suction press roll, and two grooved press rolls. A steam box heats the pulp mat just prior to the couch roll. The pulp mat is dried to its final air dry consistency with a 21-deck airborne Flakt dryer, and then sheeted and baled.

The temperature measured at the headbox is about 153°F (about 67.2°C) and the pH measured in the whitewater pit is between about 4.0 and about 4.1. The fourdrinier wire speed is 486 ft/min.

A defoamer, which is a 10% actives blend of C16 and Ci3 alcohols is added to the whitewater pit at a level of 0.5 lb product/T and increased to 2 lb product/T over the next 90 minutes. An anionic surfactant, which is 30% actives sodium dodecyl sulfate, is added at the inlet side of the fan pump and dosed between the range of about 0.25 lb/T and about 0.65 lb/T. The trial is conducted without making any adjustments to the stock flow rate, machine speed, or steam valve so that changes caused by the anionic surfactant are observed. After the anionic surfactant is dosed to the machine, the following changes are observed: The dry line moves back toward the headbox, and remains 1.5 feet closer to the headbox, even after reducing some of the early table vacuums.

The couch vacuum (the couch roll is at the end of the fourdriner, so its place is after the pulp has had a vacuum pulled on it) gage shows an increase in vacuum from 18.2. to as much as 20.0 in. Hg.

The draw tightens between the couch and first press requiring an adjustment to maintain proper tension.

The couch solids (couch solids are those solids present after vacuum dewatering) increase according to two independent measurements. Both gamma backscatter and gravimetric consistency measurements show a couch consistency increase of better than two points. The gravimetric couch consistency improves from 16.4 to 18.9% and the gamma backscatter consistency improves from 26.0 to 28.1%.

Sheet moisture at the dry end cutter of the machine decreases from 17.2 to 13.9%.

The air-dry consistency of the sheet at the dry end cutter increases from 92.1 to 95.7%.

Plant Trial Example 2 2nd Trial at Roadrunner Pulp Mill in Irontown, MI A second trial is held at the Roadrunner Pulp Mill with the goal of increasing pulp vacuum dryer production as a result of improving drainage and drying.

The conditions for the plant trial are the same as for Plant Trial Example 1 except that the pulp flow rate, machine speed and dryer steam valve are adjusted to allow for increased production.

The first part of the trial examines whether an improvement in drainage in a vacuum dryer can be obtained by just adding defoamer. From 0 to 3.6 lb product/T of defoamer (the same defoamer as in Plant Trial Example 1) is added to the pulp and no significant drainage is achieved by addition of defoamer alone. The conclusion reached is that the defoamer, by itself, does not enhance vacuum dewatering of pulp.

The second part of the trial is conducted by adding a mixture of the defoamer (dose of about 2 lb product/T) and surfactant (dose of from about 0.25 lb/T and about 0.75 lb/T) from Plant Trial Example 1. During the trial period the rate of production on the pulp dryer is increased significantly.

During the trial period, wire speed increases from 484 feet per minute (hereinafter"FPM") to 503 FPM (19 FPM).

The pulp rate is over 5000 gallons a minute. Upon addition of the mixture of anionic surfactant and defoamer the pulp rate increases by 70 to 100 gallons per minute. This is a desirable increase.

Steam valve position decreases from 100 to 80% as mixture of anionic surfactant and defoamer is added to the process. Steam valve opens back to 100% as production is increased.

Dry line moves significantly (-50 inches) toward the headbox as mixture of anionic surfactant and defoamer is added. First press draw is adjusted 0.1 to slacken draw at the couch. The slice is opened 0.2 to control the dry line. Orthos, flatboxes, and couch vacuums all increase significantly during the trial and decrease slowly after the trial ends. Response time to mixture of anionic surfactant and defoamer addition is quick.

The air dry tons per hour of pulp produced increases from a pre-trial average of 52.4 to a trial period high of 54.1. This amounts to an increase of 1.7 tons of pulp per hour, which is equivalent to 40.8 more tons of pulp per day. This increase in pulp production is of great benefit to the pulp producer.

Changes can be made in the composition, operation and arrangement of the method of the present invention described herein without departing from the concept and scope of the invention as defined in the following claims: