WO2008076055A1

WO2008076055A1 - Process of pulping

Info

Publication number: WO2008076055A1
Application number: PCT/SE2007/050900
Authority: WO
Inventors: Patrik Johansson; Sofia Enberg; Pär NILSSON; Marie R. Samuelsson; Pia Hellström
Original assignee: Akzo Nobel N.V.
Priority date: 2006-12-19
Filing date: 2007-11-26
Publication date: 2008-06-26

Abstract

The present invention relates to a process of producing pulp comprising digesting lignocellulose-containing cellulosic fibers in a digestion liquid, wherein the fibers are treated in the presence of at least one chelating agent capable of forming complexes with metal ions in an amount from about 0.5 to about 10 kg/ton dry fiber material, and at least one surfactant in an amount from about 0.1 to about 10 kg/ton dry fiber material. The invention also relates to a pulp obtainable by the process.

Description

Process of pulping

The present invention relates to a process of pulping, particularly aimed to reduce the contents of undesired organic and inorganic substances to facilitate closure of mills, improve brightness and strength properties of the obtained pulp.

Background of the invention

The principal steps of pulping including addition of conventional cooking chemicals to cellulosic raw material are well-known in industrial pulpmaking. Thereafter, the pulp is processed in conventional manner for production of an end product e.g. fine paper, newsprint, paperboard etc. Such subsequent process stages may include inter alia washing, bleaching, beating, and formation of a paper web on a wire in the papermaking process. In the pulping process, various chemicals may be added to impart desired properties to the pulp, e.g. by removal of undesired substances. US 5,593,544 discloses a pulping process in which chelating agents are added in order to remove metals from the cellulosic material. However, this method may still be improved, particularly with regard to brightness and chemical composition

It would be desirable to provide a process enabling further closure of pulp mills and thereby cope with environmental problems while reducing the need of water supply to mills. It is thus one objective of the present invention to enhance at least partial closure of pulp mills thus reducing organic and inorganic effluents discharged from the process, particularly from bleaching sequences following the pulping stage. Recirculation of process water can then be increased which of course is of considerable importance, particularly in areas where water supply is limited. A further objective is to provide an improved pulping process increasing the brightness of the treated pulp, particularly while imparting increased rewetted zero-span to the fibers.

The invention

The present invention relates to a process of producing pulp comprising digesting lignocellulose-containing cellulosic fibers in a digestion liquid, wherein the fibers are treated in the presence of at least one chelating agent capable of forming complexes with metal ions in an amount from about 0.5 to about 10 kg/ton dry fiber material and at least one surfactant in an amount from about 0.1 to about 10 kg/ton dry fiber material.

A wide variety of lignocellulose-containing cellulosic fibers can be employed in the process derived from e.g. wood logs, finely divided raw materials, woody materials such as wood particles and fibers of annual or perennial plants, and non-wood. The woody raw material can be derived from hardwood or softwood species, such as birch, beech, aspen, alder, eucalyptus, maple, acacia, mixed tropical hardwood, pine, fir, hemlock, larch, spruce, and mixtures thereof. Non-wood plant raw material can be provided from e.g. straws of grain crops, reed canary grass, reeds, flax, hemp, kenaf, jute, ramie, sisal, abaca, coir, bamboo, bagasse, and combinations thereof.

According to one embodiment, the treatment with chelating agent is performed at a pH value ranging from about 7 to about 14, for example from about 8.5 to about 14, or from about 10 to about 14, or from about 12 to about 14. According to one embodiment, the weight ratio of liquid to dry fiber material ranges from about 2:1 to about 10:1 , for example from about 3:1 to about 10:1 , or from about 3:1 to about 8:1. According to one embodiment, said treatment is performed at a temperature from about 5 to about 180 °C, for example from about 15 to about 170 °C, or from about 100 to about 170 ⁰C.

According to one embodiment, the treatment with chelating agent is performed at an absolute pressure from about 0 to about 20, for example from about 0.1 to about 15, or from about 0.2 to about 10 bar. According to one embodiment, the treatment is performed during a period from about 5 minutes to about 5 h, for example from about 10 minutes to about 3 h, or from about 1 h to about 2 h.

According to one embodiment, the chelating agent is added to the lignocellulose- containing fiber material, such as woodchips in an amount from about 0.5 to about 5, e.g. from about 1.5 to about 5, such as from about 2 to about 4 kg/ton dry fiber material.

According to one embodiment, a separate treatment vessel may be used for treatment of the cellulosic fibers with chelating agent located before, i.e. upstream of a digester tank containing the digestion liquid. According to one embodiment, digestion may be performed in a continuous or a discontinuous process.

According to one embodiment, a considerable portion, e.g. at least about 90 % by weight, of the metal complexes formed is removed from the digestion liquid upon completion of the treatment. This can be achieved by draining, e.g. thickening, and subsequent washing of the fibers with a liquid free from metals or having low metal content.

According to one embodiment, the digestion liquid consists of effluents recycled from the pulp mill. Such effluents may comprise spent liquor, fresh digestion liquid, effluent from bleaching processes, condensation, mains water or lake water, and mixtures thereof. The spent liquor used is suitably the spent liquor having reduced, low content of metals obtained at said digestion. The spent liquor may be black liquor received from the digestion of lignocellulose-containing fibers that have been treated with chelating agent.

According to one embodiment, the digestion process includes a pre- impregnation of the fibers with digestion liquid and/or spent liquor. According to one embodiment, treatment with chelating agent is performed prior to said pre-impregnation which may be followed by a washing stage. According to one embodiment, treatment with chelating agent is performed in combination with the actual pre-impregnation as an integrated treatment, in which the chelating agent may be added together with an impregnation liquid.

According to one embodiment, chelating agents able to form complexes with metals may be selected from e.g. non-nitrogenous polycarboxylic acids such as oxalic, citric or tartaric acid; nitrogenous polycarboxylic acids such as diethylene triamine pentacetic acid (DTPA), ethylene diamine tetracetic acid (EDTA), nitrilo triacetic acid (NTA), triethylenetetramine hexa acetic acid (TTHA); phosphonic acids or derivatives of such acids including morpholinomethylenebisphosphonic acid (MMBA), bis(aminomethyl) dicyclopentadiene tetra(methylenephosphonic acid), bis(aminomethyl),bicycloheptane tetra(methylenephosphonic acid), ethylenediamine tetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid)(DETA-PMP), pentaethylenehexamineocta (methylenephosphonic acid), hexamethylenediamine tetra(methylenephosphonic acid), piperazinedimethylenephosphonic acid and phosphonomethylated polyalkylene polyamines, particularly such having molecular weights up to about 100,000 or more, which may contain piperazine rings in the chain, polyaminopoly(methylenephosphonic acids) including ethylenediaminetetra (methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid) (DTPMP), triethylenetetraminehexa (methylenephosphonic acid)(TTHMP), hexaethyleneheptaminenona(methylene phosphonic acid), polymethylenephosphonic acid derivatives of polypropylenepolyamines such as di-1 ,2- propylenetriaminepenta(methylenephosphonic acid), and mixtures thereof.

Further examples of suitable chelating agents include dicyclopentadiene and bicycloheptane derivatives containing dimethyltricyclodecane and dimethylnorbornane radicals respectively. Further examples of chelating agents may be any of those disclosed in US 4,799,995, US 2003/0010458, and US 2002/0139497 A1.

According to one embodiment, surfactants such as non-ionic, cationic, anionic surfactants, and mixtures thereof may be used. According to one embodiment, anionic surfactants can be naphthalene sulphonate and derivatives thereof and phosphate compounds such as phosphate esters, and mixtures thereof.

According to one embodiment, naphthalene sulphonates having alkyl groups of 1-10 carbon atoms or 1-3 carbon atoms, such as methyl, isopropyl, n-butyl, sec-butyl and nonyl may be used. According to one embodiment, sodium butyl naphthalene sulphonate and sodium nonyl naphthalene sulphonate may be used.

According to one embodiment, naphthalene sulphonate-formaldehyde condensate or alkyl naphthalene sulphonate-formaldehyde condensate of a sodium salt having a mean molecular weight of e.g. from 300 to 2000, such as from 400 to 1000, or from 500 to 750 may be used. When present, the alkyl groups suitably contain 1-3 carbon atoms.

According to one embodiment, the anionic surfactants can be selected from phosphated compounds including phosphated alcohols or phosphated alcohol alkoxylates obtained by different processes, the most common being the reaction of an alcohol or alkoxylated alcohol with polyphosphoric acid or phosphorous pentoxide (P₂O5).

The starting alcohols may be aliphatic or aromatic, linear or branched, saturated or unsaturated. The alcohols may be alkoxylated with ethylene oxide (EO) and/or propylene oxide (PO). If the alkoxylation is performed with both EO and PO, the ethyleneoxy units and the propyleneoxy units can be added randomly or in blocks. The blocks could be added to the alcohol in any order.

When P₂O₅ is used as the phosphatising reagent, the molar ratio between P₂O₅ and alcohol or alkoxylated alcohol may be 1 :3. The product mixture can then contain about equal amounts of monoalkylphosphate ester and dialkylphosphate ester, and only smaller amounts of inorganic phosphate residues. A larger amount of alcohol or alkoxylated alcohol will yield more diester, and a smaller amount will yield more monoester.

According to one embodiment the surfactant is a non-ionic surfactant. According to one embodiment, nonionic surfactants may be selected from compounds having the formula R₃O-(PO)_x(EO)y(PO)_zH, wherein R₃ is a C₈ to C₂₄ hydrocarbyl group, such as C₈ to Ci₈, or an alkylaryl group having one or two alkyl substituents with 4-12 carbon atoms, such as 9-12; PO is a propyleneoxy unit, EO is an ethyleneoxy unit, x=0-5, such as 0-4, e.g. 0-2; y=1-50, when R₃ is an alkyl or alkenyl group y is e.g. 10-20, and when R₃ is an alkylaryl group, y is e.g. 4-12; and z=0-5, such as 0-4, or 0-2 or 0. Thus, the alcohol alkoxylates may, in addition to the 1-50 ethyleneoxy units, also contain propyleneoxy units. The number of propyleneoxy units, when present, may be as small as 0.1 mole PO per mole alcohol. According to one embodiment, the propyleneoxy groups are located next to the

R₃O group. The ethyleneoxy units and the propyleneoxy units could be added randomly or in blocks. The blocks could be added to the alcohol in any order. The alkoxylates may also contain an alkyl group with 1-4 carbon atoms in the end position. According to one embodiment, the alkoxylates contain 4-20 ethyleneoxy units and 0 propyleneoxy units. The hydrocarbyl group of the nonionic surfactants may be linear or branched, saturated or unsaturated. Suitable linear nonionic surfactants are based on Cg-Cn alcohol, dodecanol, tridecanol, tetradecanol, Ci₀-Ci₄ alcohol and oleyl alcohol that are ethoxylated with 10-20 moles of EO. Suitable branched nonionic surfactants are 2-ethylhexanol, 2- propylheptanol, isodecanol and 2-butyloctanol which are ethoxylated with 10-20 moles of EO. An example of a nonionic surfactant with an alkylaryl group is dinonyl phenol comprising 15-24 moles of EO.

Wherever the degree of alkoxylation is discussed, the numbers referred to are molar average numbers. According to one embodiment, the non-ionic surfactant may have the formula

R(AO)χ(C₂H₄O)yH, where R is a hydrocarbyl group R'O- having 8 to 24 carbon atoms or a group R¹¹CONR'"-, where R" is a hydrocarbyl group having 7 to 23 carbon atoms, R'" is hydrogen or the group -(AO)χ(C₂H₄O)_yH, AO is an alkyleneoxy group with 2-4 carbon atoms, x is a number from 0 to 5 and y is a number from 1 to 50. These alkoxylates could contain a hydrophobic group of 8-24 carbon atoms, which may be an alkyl group or an acyl group containing from 8 to 24 carbon atoms, or a hydrocarbyl group which could be an alkylaryl group having alkyl group substituents with 4-12 carbon atoms. The alkyl or acyl group of the nonionic surfactants may be linear or branched, saturated or unsaturated. Suitable examples of such nonionic surfactants are alkylene oxide adducts obtained by alkoxylation of an alcohol, an amine or an amide. One example is compounds having the formula RO(AO)₃H, wherein R' is a hydrocarbyl group having 8-24 carbon atoms, a is from 2-12, such as 3-10, and AO is an alkyleneoxy group having 2-4 carbon atoms, the number of ethyleneoxy groups being at least 50% of the total number of alkyleneoxy groups. The R' group may be branched or linear, saturated or unsaturated, aromatic or aliphatic. Examples of hydrocarbon groups R' are: 2-ethylhexyl, octyl, decyl, cocoalkyl, lauryl, oleyl, rape seed alkyl, tallow alkyl, octylphenyl, nonylphenyl and dinonylphenyl. Especially suitable hydrocarbon groups are those obtained from oxoalcohols, Guerbet alcohols, methyl substituted alcohols with 2-4 groups having the formula -CH(CH₃)- included in the alkyl chain, and straight alcohols. Another example of suitable nonionic surfactants are compounds having the formula R"N(AO)_t,iH(AO)_b2H, wherein R" is a hydrocarbyl group or an acyl group having 8-18 carbon atoms, AO is an alkyleneoxy group having 2 or 3 carbon atoms and the sum of b1 and b2 is 2-12, e.g. 3-10. The hydrocarbon group and the acyl group can be aromatic or aliphatic, branched or straight, saturated or unsaturated. Examples of suitable groups are 2-ethylhexyl, octyl, decyl, cocoalkyl, lauryl, oleyl, rape seed alkyl, tallow alkyl and the corresponding aliphatic acyl groups. If R" in the formula Vl is an acyl group, one of b1 and b2 may be 0, whereas if the nitrogen atom is an amine nitrogen, b1 and b2 may be different from zero.

According to one embodiment, non-ionic surfactants include e.g. ethoxylated alcohols, for example R-O(CH₂CH₂O)_n-H, where R=alkyl or alkenyl group, n=1-50, for example 10-20, ethoxylated dialkylphenols, where R=nonyl group, n=1-50, for example n=15-24. According to one embodiment, ethoxylated alcohols include ethoxylated oleyl alcohols (R of CH₃(CH₂)₇ CH=CH(CH₂)₈) and ethoxylated isostearyl alcohols.

According to one embodiment, non-ionic surfactants include condensation products of alkylene oxides and alkyl phenols. Thus, useful non-ionic surfactants are, e.g. polyethoxyalkylphenols wherein the alkyl group contains 4 to 12 carbon atoms and wherein the number of ethylene oxide (EO) groups condensed with the alkyl phenol may range from 6 to 15.

Other nonionic surfactants include naturally occurring surfactants such as mixed alkyl glucosides and oligosaccharides. The mixed alkyl glucosides also benefit from the formulation by providing a homogeneous, substantially neutral pH mixture in combination with e.g. ethylene glycol, glycerine and/or other polyhydroxy compounds.

According to one embodiment, the surfactant is added in an amount from about 0.05 to about 5, for example from about 0.5 to about 5 kg/ton, or 0.1 to about 2 kg/ton dry fiber material. According to one embodiment, other naturally occurring compounds, such as lignin and carboxylic groups in the carbohydrates may be added to the digestion liquid. Since it is undesirable for the released metal ions to become attached or re-attached to these organic compounds and be retained in the pulp stream, the effectiveness of the metal removal treatment is dependent upon the addition point of the chelating agent. If the chelating agent is present at a point where the lignin concentration or carboxylic group concentration is high, the metal removal may not be as effective as if the chelating agent were present when the concentration of these compounds is lower.

The present invention may be used in the production of any type of pulp, e.g. obtained from Kraft (or sulfate), soda, alkaline sulfite and bisulfite processes; neutral sulfite pulp, pulps of anthraquinone plus hydroxide (NaOH/KOH) or carbonate (Na₂CO₃/K₂CO₃) plus possibly oxygen gas, polysulfide pulp, pulp produced by pre- impregnating wood with hydrogen sulfide before alkaline delignification, and pulps produced by delignification of wood with an organic solvent such as methanol, ethanol, optionally in the presence of an inorganic solvent.

According to one embodiment, a batch type digester is being employed to which lignocellulose-containing fibers and a mixture of any of "weak black liquor", i.e. spent liquor from a previous digester cook, and "white liquor", i.e. a solution of sodium hydroxide and sodium sulfide, that is either fresh or recycled, is pumped into the digester. In the digestion process lignin, which binds the fibers together, is dissolved in the white liquor forming pulp and black liquor. After cooking at a particular temperature and pressure (H-factor) in the digester, the digester contents (pulp and black liquor) can be transferred to a holding tank. The pulp in the holding tank can be transferred to brown stock washers while the liquid (black liquor formed in the digester) is sent to the black liquor recovery area, i.e. black liquor evaporators. The black liquor may be evaporated to a high solids content, usually 60 to 80 wt% solids, using a multiple effect evaporator. The higher the solids content, the more difficult it is to pump the black liquor and the more scale problems the pulp mill will have. One of the most troublesome is calcium carbonate scale which forms in various areas of the pulp mill, including the digester, the black liquor evaporator area, and the brown stock washing area.

According to one embodiment, the digester composition can contain a large amount of sodium sulfide, which is used as an accelerant to increase the delignification rate of the cook. This works to release the lignin in the wood chips and thus the cellulose becomes available as pulp.

According to one embodiment, the pulp is delignified with oxygen gas after the digestion process. The pulp delignified with oxygen gas may then suitably be bleached, e.g. with a bleaching agent containing hydrogen peroxide, chlorine dioxide, ozone, and/or peracetic acid in any combination.

According to one embodiment, effluents from the digesting and bleaching stages are recycled to the digesting stage.

The invention further relates to a pulp obtainable from the process as defined herein. The invention being thus described, it will be obvious that the same may be varied in many ways. The following examples will further illustrate how the described invention may be performed without limiting the scope of it.

All parts and percentages refer to part and percent by weight, if not otherwise stated. Example

Commercial Scandinavian softwood, 80% pine and 20 % spruce, was dried at

40°C to approximately 95% dry content and fractioned according to SS 18 71 74:2. The selected fraction (15-45 mm in diameter) was sorted by hand and knots and bark were removed. The chip dry content was analyzed according to SCAN-CM39:94. Each run was performed using 1.6 kg absolutely dry non-steamed woodchips.

The trials were performed at a weight ratio of wood to cooking liquor of 1 :5. The chemical charge of cooking chemicals was made so as to result in effective alkali (EA) of 22% and sulphidity (S) of 35%. Additives were charged in conjunction with the cooking chemicals in the beginning of the impregnation phase. The impregnation phase was performed at an increasing temperature starting from 100°C and rising to 165 °C with a rate of 1.5 °C/min. The temperature was then maintained at 165°C for about 100 minutes to reach an H-factor of 1 100 (expressing the average degree of delignification during digestion). After completed digestion the black liquor was removed and the pulp was rinsed with cold water. The pulp was disintegrated according to ISO 5263:1995 and washed before screening (Tappi 275sp-02). Laboratory sheets were prepared from the screened pulp according to ISO 3688-1977. These sheets were analyzed for viscosity (SCAN- CM15:99), kappa number (ISO 302), and ISO brightness (ISO 2470). Metals were analyzed with ICP (inductively coupled plasma). The pulps were pre-processed in a PFI beater according to ISO 5264 for further physical testing including tensile properties SCAN-P67:93, zero-span dry and rewetted (ISO 15361 ). Sheets for physical testing were prepared according to ISO 5269.

The results are presented in table 1 as a comparison between pulp produced without additive charge and pulps produced with charge of additives including addition of a sole chelating agent (5kg/ton dry wood chips) and addition of chelating agent (5 kg/ton dry wood chips) and surfactant (5 kg/ton dry wood chips). All results are given as a change in absolute value in their respective units for the property measured by comparison with results for no additive charge. Table 1

From the above results, it can be particularly noted that brightness change was +2.4 (i.e. 2.4 units higher than results for pulp without any charge of chelating agent and surfactant) for combined addition of surfactant and chelating agent compared to +1.2 for sole addition of chelating agent. Also, it can be noted that rewetted Zero span tensile index was considerably increased in pulp to which combined addition according to the invention was made compared to sole addition of chelating agent.

Claims

1. Process of producing pulp comprising digesting lignocellulose-containing cellulosic fibers in a digestion liquid, wherein the fibers are treated in the presence of at least one chelating agent capable of forming complexes with metal ions in an amount from about 0.5 to about 10 kg/ton dry fiber material, and at least one surfactant in an amount from about 0.1 to about 10 kg/ton dry fiber material.

2. Process according to claim 1 , wherein the fibers are derived from softwood.

3. Process according to claim 1 or 2, wherein the chelating agent is selected from non-nitrogenous polycarboxylic acids, nitrogenous polycarboxylic acids, phosphonic acids, and mixtures thereof.

4. Process according to any one of claims 1 to 3, wherein the surfactant is anionic.

5. Process according to any one of claims 1 to 4, wherein the surfactant is added in an amount from about 0.5 to about 5 kg/ton dry fiber material.

6. Process according to any one of claims 1 to 5, wherein the chelating agent is added in an amount from about 1.5 to about 5 kg/ton dry fiber material.

7. Process according to any one of claims 1 to 6, wherein the temperature in the digestion liquid ranges from about 5 to about 180 ⁰C.

8. Process according to any one of claims 1 to 7, wherein the pressure in the digestion liquid is from about 0.2 to about 10 bar.

9. Process according to any one of claims 1 to 8, wherein the retention time in the digestion liquid is from about 5 minutes to about 5 hours.

10. Process according to any one of claims 1 to 9, wherein the anionic surfactant is naphthalene sulphonate.

1 1. Process according to any one of claims 1 to 10, wherein the pulp is subsequently bleached.

12. Process according to any one of claims 1 to 1 1 , wherein effluents from the process are recycled to the digestion liquid.

13. Pulp obtainable by a process according to any one of claims 1 to 12.