Pulping and Bleaching PSE 476/Chem E 471 Lecture #18 Bleaching Fundamentals and Bleaching Sequences PSE 476: Lecture 18

Bleaching Sequences Agenda Bleaching chemistry fundamentals Cationic mechanisms Radical mechanisms Anionic mechanisms Bleaching chemicals nomenclature rules Typical Sequences Practice PSE 476: Lecture 18

Bleaching: Basic Chemical Principles Bleaching reactions can be divided into anionic, cationic (?) and radical reactions. There are a multitude of different reactions that occur during bleaching. We are going to cover the chemistry in very generic terms. PSE 476: Lecture 18

Bleaching Mechanisms PSE 476: Lecture 18

Cationic Reactions Cationic reactions occur under acidic conditions. Reactive species: Chlorine: Cl+ Peroxy acids: OH+ Ozone: O3 How can these species be cations??????? Electrophilic substitution reaction: PSE 476: Lecture 18

Cationic Reactions: Generalities (2) The most important reaction is electophillic substitution onto the aromatic ring. There can be multiple substitutions. Substitution at #1 ring position can later cause side chain elimination. Substitution at #3 or #4 position can lead to the formation of an ortho quinone. There is no ring cleavage. PSE 476: Lecture 18

Radical Reactions: Generalities Nobody adds free radicals to a bleaching reaction; they are formed from other additives. Examples: Chlorine: Cl• (chlorine radical) Oxygen: -O2• (superoxide radical) HO• (hydroxyl radical) Many free radicals react very rapidly with lignin so they are good except that they degrade carbohydrates very rapidly so they are also bad. PSE 476: Lecture 18

Radical Reactions: Generalities (2) Pathways are pH dependent. Free radicals couple with aromatic rings. This and further mechanisms affected by free versus etherified phenolic hydroxyls. Results in generation of free radical structures. These structures can undergo additional substitutions and oxidation reactions. Generation of ortho quinones. Ring opening reactions. Ring substitution by bleaching agent. Some side chain cleavage. Carbohydrates will react and are degraded. PSE 476: Lecture 18

Radical Reactions: Generalities Ring Cleavage Ortho Quinones Side Chain Cleavage Ring Substitution PSE 476: Lecture 18

Anionic Reactions: Generalities These are alkaline bleaching reactions. ClO -, HOO – added reagents Other species generated: . O2-, . O-, etc. These chemicals are involved in nucleophilic attacks. Target of attacks: carbonyls and conjugated carbonyls. Products of reactions: Ring opening: Dicarboxylic acids Side chain cleavage PSE 476: Lecture 18

Anionic Reactions: Generalities oxirane PSE 476: Lecture 18

Bleaching Sequences Nomenclature Rules (1) Nomenclature rules found in TAPPI Information Sheet TIS 0606-12. Bleaching sequences are described by capital letters which describe the chemicals used. C = chlorine, H = hypochlorite, D = chlorine dioxide, O = oxygen (with NaOH), P = peroxide, Z = ozone, Y = hydrosulfite, X = enzymes, E = NaOH, Q = chelants, Paa = peracetic acid Letter represents each chemical followed by a washing step. CED = Cl2/wash/NaOH extraction/wash/ ClO2/wash PSE 476: Lecture 18

Bleaching Sequences Nomenclature Rules (2) If washing is omitted, a parenthesis is added around the steps. C(EO)D = Cl2 /wash/NaOH/Oxygen/wash/ClO2/wash If chemicals are added simultaneously, the letters are placed in parenthesis separated by a + sign. C(E+O)D = Cl2 /wash/NaOH+Oxygen/wash/ClO2/wash People will also list this as CE/OD Sometimes small amounts of chemicals are added to fortify a step. The additives are typically listed as subscripts: EO or EP or EOP Subscripts are also used to designate differences in conditions (ie temp, pH, etc): D0, D1, D2 PSE 476: Lecture 18

Differences in Chlorine Dioxide Stages Condition D0 D1 D2 Final pH 1.5-2.5 3.5-5 3.5 - 5 Temp °C 40°C - 60°C 55°C - 75°C 60°C - 85°C Consistency 10-15% Time 30-80min 2-4 hours Charge 1-2 times kappa # 4-6 times kappa # PSE 476: Lecture 18

Bleaching Sequences Typical Bleaching Sequence (1990) PSE 476: Lecture 18

Bleaching Sequences Traditional Sequences PSE 476: Lecture 18

Bleaching Sequences Some Current (2001) Sequences PSE 476: Lecture 18

Elemental Chlorine Free ECF pulping signifies that there is no elemental chlorine (Cl2) used in the bleaching sequence. Most often oxygen or ClO2 is used for lignin removal. Brightening is most often accomplished by using DED. Peroxide is used mainly to reinforce oxygen or in the extraction stages. PSE 476: Lecture 18

Total Chlorine Free TCF means that there is no chlorine of any kind used in the bleaching process. There has been significant effort made in this direction for the following reasons: There is no possibility of the generation of AOX (AOX = Absorbable Organically Bound Halogens). Chlorine free bleach plant effluent can be burned thus allowing a pulp mill to have basically no effluent. The problem to date with TCF is that it has not been possible to obtain desired brightness without significant strength loss. PSE 476: Lecture 18

Bleaching Sequences Name the Sequence (1) O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2 Cl2/wash/NaOH/wash/NaOCl Cl2 & ClO2 /wash/NaOH/wash/NaOCl/wash/ClO2 /wash/NaOH/wash/ClO2 O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2 O2/wash/EDTA/wash/NaOH then H2O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2 PSE 476: Lecture 18

Bleaching Sequences Name the Sequence (2) Can you name this sequence? Hint: Oxygen bleaching (O) is typically carried out in only the first phase. This allows the effluent to be sent to the recovery system (no possibility of Cl compounds) PSE 476: Lecture 18

Bleaching Sequences Agenda Bleaching chemistry fundamentals Cationic mechanisms Radical mechanisms Anionic mechanisms Bleaching chemicals nomenclature rules Typical Sequences Practice PSE 476: Lecture 18