METHOD FOR SULFITE PULPING USING SURFACTANTS BACKGROUND OF THE INVENTION The sulfite pulping of wood is a well known process and is described in Pulp and Paper Chemistry and Chemical Technology, Volume 1, Chapter 4, edited by James P. Casey, Wiley Interscience 1980, the disclosure of which is incorporated herein by reference. Sulfite pulping is similar to Kraft (i. e., sulfate) pulping in that the pulp yield and reject level are a function of the degree of delignification. The delignification process depends on the penetration and diffusion of cooking chemicals in the wood substance. Usually, the cooking liquor moves much more rapidly in the longitudinal direction than in the transverse direction of the fibers; with more rapid heating, cooking tends to be uneven. Insufficient penetration and diffusion of cooking liquor causes the centers of wood chips to be undercooked or burnt. As a result, higher screen rejects and lower yields are obtained.

Satisfactory sulfite pulping, especially the acid sulfite process, can be achieved most readily if the cooking liquor is brought into intimate contact with the wood chips. Free sulfur dioxide in acid sulfite liquor

enters wood more rapidly than does combined sulfur dioxide; therefore, the centers of chips may sometimes contain a sulfur dioxide solution but no base. At temperatures above about 110°C, such an occurrence is expedited, and chips become brown (due to lignin thermal condensation) at the center, and are soon rendered uncookable. In order to avoid this occurrence, presteaming is often utilized to improve the penetration of cooking liquor. In addition to presteaming, penetration and diffusion can also be facilitated by hydrostatic pressure and by increasing chip moisture content.

Many studies have been conducted to understand the cooking mechanisms and to improve the efficiency of the cooking process. A number of digester aids were found to be highly effective in enhancing the efficiency of the sulfite process. These chemicals include various quinone derivatives, dimethylamides, esters of formic acid, and amphoteric surfactants.

Esters of formic acid (i. e., formates) function as inhibitors for the condensation or polymerization of the lignin during pulping, which in turn improves the cooking process. Dimethylamides and amphoteric surfactants are surface active agents, and function as penetrating and dispersing agents to enhance cooking liquor penetration.

Several factors are believed to be responsible for the mechanisms of cooking liquor penetration, including interfacial tension, wettability, and emulsification. The interfacial tension between the cooking liquor and resin must be decreased in order to increase the penetration rate of cooking liquor into the wood chips. There are two mechanisms

corresponding to the lowering interfacial tension: Deformation of resin and formation of a resin-in-water mulsion.

Low interfacial tension reduces the work of deformation necessary for resin droplets to emerge from the narrow necks of pores. A low water/resin interfacial tension is desirable for the movement of resin through the narrow spaces. This mechanism demonstrates how the cooking liquor can penetrate deeply into the chips.

Alternately, a very low interfacial tension is needed to form an mulsion of resin in the cooking liquor. If resin, which blocks the pores, can be emulsified by a surface active agent, the cooking liquor can easily pass through the pores. This leads to a good cooking liquor penetration, while also preventing redeposition of dissolved resin particles back onto the fibers.

The increased wettability of wood chip surfaces by a surface active agent also creates more favorable conditions for cooking liquor penetration. The spreading of cooking liquor on the chip surface is governed by the surface tension of cooking liquor, the surface tension of the chip, and the interfacial tension between the cooking liquor and the chip. In general, the lower the surface tension of the cooking liquor, the easier the spreading occurs.

Free sulfur dioxide in acid sulfite liquors enters wood more rapidly than does combined sulfur dioxide. It is desirable to facilitate cooking liquor penetration in a more uniform, thorough pattern. An increase in the moisture content of wood chips and the wetting of wood chip surfaces will

alleviate non-uniform penetration. The addition of surface active agent in the cooking liquor can further enhance the penetration mechanism.

The present invention demonstrates that surfactants with high HLBs (where such measurements are appropriate) are effective in facilitating the spreading of cooking acid on wood chip surfaces. HLB is an abbreviation for hydrophile-lipophile balance as related to the oit and water solubility of a material. A high HLB indicates that the hydrophilic portion of the molecule is dominant, while a low HLB indicates that the hydrophobic portion of the molecule is dominant. The water solubility of materials increases with increasing HLB.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved method for sulfite pulping of wood chips in which wood chips are digested in an acid sulfite pulping liquor to produce a wood pulp, the improvement comprising adding to the chips, prior to mixing with the acid sulfite pulping liquor, an effective amount of a nonionic or anionic surfactant, or mixtures thereof, the method enhancing the penetration of the acid sulfite pulping liquor into the chips. The treatment of the present invention demonstrates efficacy as a digester aid for sulfite pulping by improving the penetration of cooking, liquor via emulsification and wetting mechanisms.

In a preferred embodiment, the present invention allows for the use of ethoxylated surfactants as digester aids for acid sulfite pulping processes. The present invention reduces reject levels and/or provides higher yield production rates by reducing cooking time, cooking chemicals and/or cooking temperature.

Nonionic ethoxylated surfactants, e. g., linear alcohol ethoxylates, alkylphenol ethoxylates and EO/PO block copolymer surfactants, as well as anionic surfactants, e. g., alcohol ether sulfates, phosphate esters, ether phosphates, etc., are all effective for purposes of the present invention. Dosage ranges of from about 0.1-10 pounds of treatment per ton of oven dry wood chips are considered effective, with about 0.25-5 pounds/ton being preferred.

An HLB range for the nonionic surfactant treatment of from about 10-15 is preferred. A surfactant with an HLB below about 8 is oil soluble, and will not be effective for purposes of the present invention.

Particularly preferred nonionic surfactants include nonylphenol ethoxylates and oleyl alcohol ethoxylates.

The following laboratory results demonstrate the effectiveness of the present invention on the reduction of pitch-water interfacial tension and an increase in pitch dispersability.

The contact angle of surface active agents was measured using a dynamic contact angle analyzer. The wettability of the surface active agents is associated with the degree of the contact angle. The smaller the contact angle, the better the liquid spreads on the solid surface.

The dynamic interfacial tension was measured using a Drop Volume Tensiometer. Mineral oil was employed as a model system to simulate pitch components such as fatty acids, resin acids, etc.

The capability of chemical agents to emulsify pitch particles was tested by emulsifying pitch particles in a brown stock washer filtrate, which was obtained from an acid sulfite mill. The pitch particles (greater than 10 mm) were organic deposits originally collected from the sulfite mill. The effectiveness of the chemical agents was determined by measuring the amount of pitch being removed (i. e., washed out), turbidity of the mulsion, and the degree of pitch deposition on the surface.

Several surface active agents were tested. A description of these materials is found in Tables I and 11, below.

Table I Avg. Mol. Wt.

Sample Chemical Name (g/mol) <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> Component (Comp.) 1 Component 2 Comp. 1 Comp. 2<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> 1 Unsaturated alcohol Unsaturated alcohol ethoxylate ethoxylate 1148 1060-1104 2 Alkylphenol ethoxylate Alkylphenol ethoxylate 484 748 3 Branched alcohol ethoxylate 862 4 Secondary alcohol ethoxylate 834-890 5 Polyoxyethylene tridecyl ether phosphate 590-1082 6 Linear alcohol ethoxylate 666 7 Linear alcohol ethoxylate 714-742 8 Alcohol ether sulfate 400-428 9 Linear alcohol ethoxylate Linear alcohol ethoxylate 666 714-742 10 Unsaturated alcohol Secondary alcohol ethoxylate ethoxyiate 1148 834-890 11 Polyoxyethylene tridecyl Alcohol ether sulfate ether phosphate 590-1082 400-428

Table II Sample Chemical Name HLB Value Component 1 Component 2 comp. 1 Comp. 2 1 Unsaturated alcohol Unsaturated alcohol ethoxylate ethoxylate 15.3 11.3 2 Alkylphenol ethoxylate Alkylphenol ethoxylate 10.9 14.1 3 Branched alcohol ethoxylate 15.3---- 4 Secondary alcohol ethoxylate 15.4---- 5 Polyoxyethylene tridecyl etherphosphate 6 Linear alcohol ethoxylate 13.0---- 7 Linear alcohol ethoxylate 14.5---- 8 Alcohol ether sulfate 9 Linear alcohol ethoxylate Linear alcohol ethoxylate 13.0 14.5 10 Unsaturated alcohol Secondary alcohol ethoxylate ethoxylate 15.3 15.4 11 Polyoxyethylene tridecyl Alcohol ether sulfate ether phosphate The contact angle of cooking acid (pH 1.5) on Teflon@ was 112 degrees (see Table 111, below). This indicates that the cooking acid cannot wet a hydrophobic surface (e. g., aged wood chip) very well. In order for the cooking acid to spread on the wood chips quickly and completely, the contact angle of cooking acid should be reduced. As shown in Table III, all tested surfactants were able to reduce the contact angle from 112° to 66° or lower. Thus, the spreading of cooking acid on wood chip surfaces will be significantly improved with the addition of these surfactants.

Table III Contact Angle of Cooking Acid Containing Various Surface-Active Agents on Teflon Surface Contact Angle on Teflon Sample Concentration (PPm) (Dearees) Untreated------112.34 1 100 66.42 2 100 18.18 3 100 61.71 4 100 64.39 5 100 28.29 6 100 62.98 7 100 51.39 8 100 50.00 9 100 51.59 10 100 66.41 11 100 33.19 A sulfuric acid solution (pH 1.5) was used as a cooking aid to evaluate the effect of various surfactants on the reduction of interfacial tension (IFT) at the oil-cooking acid interface. Results as shown in Table IV below demonstrated that these chemicals were very effective in reducing the IFT. The IFT decreased from 48 dynes/cm (untreated) to less than 10 dynes/cm (treated). The lower the IFT, the easier the formation of an oil-in-water mulsion. It is expected that these chemicals will function effectively during the pulping process in terms of emulsifying and dispersing fatty/resin acids. As previously discussed, the reduction of IFT at the oil-cooking acid interface will enhance the penetration of cooking acid into the wood chips, while preventing redeposition of dissolved resin particles back onto the fibers. This latter aspect will improve the efficiency of the brown stock washing process.

Table IV Dynamic Interfacial Tension at Oil-Cooking Acid (pH 1. 5) Interface Dynamic Interfacial Tension <BR> <BR> (dyne/cm)<BR> <BR> <BR> <BR> <BR> Sample Concentration (ppm) Infusion Rate at: 0.2 ml/hr. 0.5 ml/hr. 1 mt/hr.

Untreated------47.87 47.97 48.18 1 100 6.27 7.44 8.82 2 100 7.63 8.55 9.60 3 100 8.22 8.74 9.79 4 100 6.61 7.56 8.66 5 100 6.20 7.50 9.16 6 100 6.31 7.93 9.44 7 100 4.08 5.17 5.44 8 100 7.45 8.33 9.86 9 100 6.36 8.10 9.54 10 100 6.21 7.61 8.99 11 100 5.47 7.31 8.96 Table V summarizes the turbidity measurements of the pitch mulsion, the percentage of pitch being washed out via 100 mesh sieve, and the pitch deposition tendency on the Teflon surface. A higher turbidity indicates that more pitch particles are emulsified in the filtrate.

As shown in Table V, all tested chemicals produced pitch emulsions with high turbidities compared to the untreated pitch containing filtrate. This was predicted by the dynamic IFT measurement. Since turbidity value is proportional to the amount of pitch emulsified in the filtrate, more pitch particles are expected to be removed from the filtrate during the washing process. As shown in Table V, a significant amount of pitch was removed by treatment with these chemicals. In addition, no deposition on the

Teflon surface was observed for these treatments. The results demonstrate that the treatments of the present invention will prevent pitch from depositing onto a hydrophobic paper machine surface.

Table V Capability of Surfactants to Emulsify Pitch Particles Deposition % of Pitch Tendency on Sample Turbidity (NTU) Washed Out Teflon Surface Untreated 180 2.9 Heavy Deposition 1 567 34.6 No Deposition 2 1026 51.1 No Deposition 9 559 31.1 No Deposition 11 776 34.5 No Deposition It is known that most ethoxylated surfactants are unstable when subjected to the conditions of the acid sulfite/bisulfite pulping processes, because the ethylene oxide groups contained in the surface active agents are decomposed at low pHs (pH<3) and high temperatures (>100°C). Consequently, the surface active agents lose their wetting ability, as well as other functions, such as emulsification capability.

Table VI below demonstrates that after cooking at a temperature of 95°C for 15 hours, a significant reduction of surface activity of sample 2 in acid solution is observed. It is expected that the ethoxylated surfactants will decompose more rapidly at a normal sulfite pulping temperature (130- 140°C).

Table Vl Surface Tension of Surfactants in Acid Sulfite Cooking Liquor Sample 2 Surface Tension (dyne/cm) Acid Sulfite Cooking Liquor H2SO4 Solution Concentration (ppm) (PH = 1.49) _ (pH = 1.45) Before After Before After 20 35.34 43.36 34.71 38.36 50 33.12 36.97 31.00 36.04 An experiment was conducted at a sulfite mill. The mill has three digesters with a total production of 170 tons per day using a calcium based sulfite pulping process. Two of the digesters have 35 ton chip capacities, while the other digester has a 65 ton capacity. The wood species was 100% birch, which was aged for one year. The moisture level of the wood chips was approximately 32 wt. %. The cooking acid had a pH of 1.45.

Sample 2 was applied at the rate of three pounds per ton, diluting prior to injection in the steam packing line with three gallons per minute of fresh water. The diluted chemical was added during the entire presteaming process of chip filling to treat chips as they entered the digester. The chip filling time was about 25 minutes for the small digesters and 45 minutes for the third digester. Following the fill, the cover was closed and cooking acid was pumped into the digester. Acid was then precirculated prior to raising to a cooking temperature of 140°C.

The chemical addition was controlled mechanically by a series of interlocked outlets energized by the opening of the steam packing valve.

Neat chemical was injected into a stream of fresh water; this mixture was injected directly into the steam line for the steam packer. When the chip filling process was complete the steam valve closed, deenergizing the chemical pump.

A minimum of 20 minutes per cook reduction in cooking time and a 25% reduction in rejects (11.3% untreated vs. 8.48% treated) was observed after the start of the trial. The pulp mill also experienced about a 4% reduction in bleaching chemicals, mostly chlorine and caustic.

An additional experiment was conducted at a sulfite mill. The mill used magnesium bisulphite solution (pH 2-3) as the cooking acid.

Sample 1 was added to the cooking acid feed pump that supplies the digesters. The treatment was added at 2 Ibs./ton of wood chips. Pre- trial, trial, and post-trial results were compared to determine if increases in production occurred when the treatment was used. Table VII below contains pre-trial, trial and post-trial averages of blows/day and tons/blow. The data demonstrates that the pulp production did not increase as compared to pre and post trial data; this is attributable to the fact that the chemical was added directly to the cooking acid, as opposed to the treatment of the present invention.

TABLEVil Average Tons/Blow Averaae Blows/Dav Pre-trial 11.54 18.69 Trial 10.81 17.25 Post-trial 11.50 17.93 While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art.

The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.