This invention relates to a novel application of water-soluble phosphonium salts or phosphine compounds for treatment of reslushed papers containing lignocellulosic materials.

BACKGROUND ART

Today, recovered paper is used in large quantities for the production of, for example, newsprint, high brightness graphic papers and tissue paper. Depending on the final product, the blend of recovered papers will be chosen to obtain optical and strength attributes required for the desired paper grade. Though recycling process layout can vary greatly from mill to mill, a limited number of general functions are used to bring the recycled pulp to a quality level similar to that of virgin pulp. Recovered paper is first added in a repulper where, under a combination of mechanical and chemical actions, paper flakes are progressively and eventually broken down to individual fibers in a re- slushing step. Concurrently to this re-slushing step in the pulper, printing inks, glues and other converting chemicals present in recovered papers are detached from paper surface and then released in the water phase. The separation of ink and other contaminants in water from the valuable fibers is then accomplished in a series of steps such as screening, cleaning, flotation and washing. In order to reach the target brightness of the product, bleaching chemicals will often be added in the repulper or later in the process, often at the end where ink particles have already been removed from the fiber suspension.

There is a definitive trend in the recycling industry to lower the manufacturing costs of deinked pulp through a reduction in deinking chemicals. This can be achieved by a change from alkaline to more neutral conditions in the repulper. However, challenging the well-proven alkaline deinking chemistry is not a simple matter. From the early days

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of flotation deinking as a technology to treat old newsprint (ONP) and old magazines (OMG) in newsprint production, repulping and flotation have been carried out in the presence of sodium hydroxide. Indeed, the use of caustic soda, calcium soaps of fatty acids and even sodium silicate was already disclosed in 1935 in the first application of flotation for printed paper stocks [Hines, P.R., US Patent 2,005, 742, 1935]. Since then, usage of caustic soda has rarely been disputed namely because the conventional alkaline deinking chemistry was not considered prohibitively expensive, and more importantly, it worked well with a wide variety of recovered papers. In newsprint production, it tends to be recognized that a final pH of 9 to 12 in the pulper favours detachment of ink via enhanced swelling of wood-containing fibre [Ferguson, L.D., "Deinking Chemistry: Part I", Tappi J., 75(7):75-83(1992)]. Moreover, at alkaline pH, calcium soaps of fatty acids are very effective because they not only promote the aggregation of ink particles but they render them more hydrophobic, thus promoting their attachment to air bubbles. For many years, there was a belief that highly alkaline conditions also promoted the hydrolysis of ester groups in print particles, which facilitated their detachment from fibre surface. This claim was however shown experimentally to be unfounded, at least if pH was below 10.5 [Ben, Y. and Dorris, G.M., "Chemical Changes in Linseed Oil under Repulping and Flotation Conditions", J. Pulp Paper ScL, 25(6):216-221(1999)].

Laboratory results as well as mill experience indicate that repulping at high pH values definitely has a number of drawbacks. [Røring, A. and Haynes R.D., "What are the Benefits and Barriers of Neutral Deinking", Progress Paper Recycling, 7(3): 73 -79 (1998); G.M. Dorris, L. Lapierre, R.D. Haynes and G. Hill, "Prospects for Near-Neutral Deiking of ONP/OMG Blends" Paper no. 14, in 6 ^th Advanced Training Course on Deinking, Grenoble, March 2003]. These disadvantages can be summarized as follows: (1) Darkening of mechanical fibres which then requires the co-addition of hydrogen peroxide and sodium silicate or chelant to regain brightness loss. This inflation of chemicals is costly. (2) High degree of fragmentation of the large sticky contaminants, which then become less effectively removed by fine screening. (3) Greater release of

dissolved and colloidal substances in the process water as pH increases. (4) Greater sensitivity to presence of flexo-printed papers because water-based inks tend to be more dispersed at higher pH and hence, more prone to redeposit in the lumen [Ben, Y. and Dorris, G.M., "Irreversible Ink Deposition during Repulping. Part I. Model Deinking Systems", J. Pulp Paper Sd, 26(3): 83-89 (2000)]. (5) Need for a substantial amount of acid to neutralize the final alkaline pulp. In North American mills where papermaking pH is still often in the neighbourhood of 5, the alkaline deinked pulp must be acidified before it is blended with virgin mechanical pulp. This acidification step not only consumes extra chemicals but it also dissolves calcium ions by decomposition of calcium carbonate from OMG coating, which is believed to promote deposition.

Though pH reduction during repulping offers many advantages, the transition to a more neutral environment during deinking of wood-containing grades of recovered paper is not straightforward because as pH is reduced, rates and extents of paper deflaking, ink dislodgement, fragmentation, redeposition and flotation are affected. Over the last few years, there has been a steady research effort in various parts of the world to overcome the limitations of a lower pH in wood-containing grades of recovered paper. This has led to a near-neutral deinking in which sodium hydroxide and hydrogen peroxide are entirely replaced by sodium sulphite. This sulphite deinking chemistry developed by Paprican, Bowater-Thunder Bay and Eka Chemicals Inc. has become a commercial reality since July 2003. A detailed description of the trial that has led to mill implementation of this technology has been presented [C. Chezick, J. Allen, G. Hill, L. Lapierre, G. Dorris, C. Castro, J. Merza and D. Haynes "A 10-Day Mill Trial of Near- Neutral Deinking. Parts I to III" in Preprints of the 7 ^th Research Forum on Recycling, pp 205-223, September 27-29, 2004]. The deinking formulation is now commercialized by Eka Chemicals Inc. under the trade-mark S-QUAD ^® . The applicability of sodium sulphite to eliminate sodium hydroxide and hydrogen peroxide from the deinking chemical recipe was triggered by some observations of Hua and Laleg [US Patent Application Publication, US2002/005999 Al, May 23, 2002] who established that

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sodium sulphite was effective for prevention of mechanical pulp darkening in the pH range of 6 to 9 of mechanical pulp containing calcium carbonate. Indeed, the idea of using sodium sulphite as a source of a weaker base than caustic soda is not new in recycling. In a 1967 Patent, Abitibi Paper Company Limited disclosed a process to deink ONP using sodium sulphite together with a soluble lactam-based polymer and a surfactant [Hossain, S.U. and Pataki, A.A., US Patent 3,354,027, November 1967]. More recently, Evans, EJ. and Gunthrie, S. of Solvay Minerals, Inc. [US Patent 5,882.476, March 1999] described a method to deink recovered paper using a mixture of sodium sulphite and sodium carbonate, in presence of a surfactant. Though several mill trials have been carried out with the Solvay chemistry, it appears that this approach is not yet a commercial reality.

Although the residual-ink content in final pulp is similar in conventional and sulphite deinking chemistries, pulp brightness after flotation is typically 2 to 3 points lower in sulphite deinking compared to conventional alkaline chemistry. To reach the target deinked pulp brightness of 58-60, typically required for newsprint, the usual post- bleaching stage of deinked pulp with sodium hydrosulphite (dithionite) entails a greater dosage with the near-neutral deinking. Moreover, the post-bleaching stage with hydrosulphite works well in most North American newsprint mills, because the pH of bleaching is often adjusted to that of the papermachine (i.e. 4.8-5.5). By lowering the pH, about one point, brightness is usually gained, even without the addition of sodium hydrosulphite. However, with the trend in North American mills to bring papermachine pH to near-neutral pH conditions, the combination of sulphite deinking chemistry and sodium hydrosulphite post-bleaching may not be able to match the performance of the alkaline peroxide chemistry. Alternative bleaching chemistry in a pulper or in a bleaching tower will then be required to reach brightness obtainable with conventional alkaline chemistry.

Hu, T.Q., James B.R., Yawalata, D., Ezhova, M.B. [US Patent Application, US 2004/0173328A1, September 9, 2004] have recently introduced a new class of bleaching and brightness stabilizing agents for virgin mechanical pulps. They found that phosphorus compounds such as tris(hydroxymethyl)phosphine (THP), tetrakis(hydroxymethyl)phosphonium chloride (THPC), and tetrakis(hydroxymethyl) phosphoniuni sulphate (THPS) have a bleaching power similar to that of sodium hydrosulphite, but can be used over a wider range of pH (4.3-9.3), temperature (20- 13O ⁰ C) and consistency (e.g. 1.5-20%), without the need for removal of oxygen or transition metal ions. However, use of these chemicals for deinking recovered paper at near-neutral pH conditions or for post-bleaching recycled fibers was not disclosed.

This class of compounds had been previously disclosed to act as a biocide during the bleaching of wood pulps with hydrogen peroxide (Bowdery, R.E, Edmunds, S. Talbot, E., US Patent Application, US 2003/0089473 Al, May 15, 2004). THPS and THP were shown to kill catalase-producing bacteria and/or catalase enzyme that hamper the bleaching action of hydrogen peroxide but the bleaching action of THPS or THP alone was not recognized.

If THPC and THPS were used today as replacement for hydrosulphite in post-bleaching of recycled pulp, economical advantages would not be achieved because these soluble phosphonium salts are now significantly more costly than sodium hydrosulphite, on an equal performance basis. However, if these phosphonium salts were used to replace sodium hydroxide, hydrogen peroxide and sodium silicate in the conventional alkaline deinking, or sodium sulphite and sodium silicate in the near-neutral deinking, then, depending on the dosage required, this family of phosphine and phosphonium type salts becomes attractive in the treatment of recovered papers with a high percentage of lignocellulosic material.

DISCLOSURE OF THE INVENTION

The purpose of this invention is to provide a deinking process that allows the brightness of final recycled pulp to approach that produced by conventional alkaline-peroxide chemistry and to exceed that of the novel near neutral process that uses sodium sulphite.

It has been discovered that addition of a phosphorus compound, more especially a water-soluble phosphine or a phosphonium salt to recycled pulp containing lignin allowed the deinking process to be carried out over a broad range of pH, from acidic to alkaline. The brightening effect of this new class of bleaching chemicals for wood- containing recycled fibers surpasses that of sodium sulphite currently used in near- neutral deinking and approaches that of the conventional alkaline peroxide chemistry.

In accordance with this invention, the phosphorous compound, more especially the phosphine or phosphonium salt, can be added directly in the pulper or at other points of the deinking process such as the dump chest or in any holding tank or bleaching tower located after ink removal steps, or in a combination of these.

Thus, in accordance with the invention, there is provided a method of recycling ink printed paper comprising repulping, deinking and bleaching the paper wherein at least one stage of the recycling is carried out in the presence of a phosphorous compound selected from a water-soluble phosphine of formula (a) or (c) or a phosphonium compound of formula (b) or (d):

or

where R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ and Rg in the formulae (a), (b), (c) and (d) are selected to provide a water solubility of at least 0.01 g/1,

m is an integer of 1 to 5, X is an anion, t is 1 or 2, k and z are the lowest possible integers such that kt = zm; and

Y and Y are independently selected from hydrogen and hydroxyalkylene of 1 to 6 carbon atoms.

In another aspect of the invention, there s provided in a process for producing a recycled pulp for paper manufacture from old ink printed papers, wherein the papers are repulped, deinked and bleached to produce the recycled pulp in the presence, in at least one stage, of a chemical agent which facilitates the recycling, the improvement wherein the chemical agent is a phosphorous compound selected from a water-soluble phosphine of formula (a) or (c) or a phosphonium compound of formula (b) or (d), as defined hereinbefore.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The brightness of recycled pulp is affected not only by the amount and brightness of fibrous and filler components but also by size and content of residual ink particles still attached to fibres or re-adsorbed on the fibres during processing steps. In order to successfully reach target brightness for a given product, both the brightness of the fibers and the effective residual ink concentration (referred to as ERIC, and expressed in ppm) must be carefully controlled. For the same amount of printed material in a recovered paper blend, ERIC will increase if a fixed mass of ink on paper releases smaller dark ink particles. Hence, conditions in repulping leading to the liberation of finer ink particles will increase ERIC and reduce brightness as these finer ink particles darken the pulp. In recycling, residual ink concentration (ERIC) and color of the lignocellulosic elements and dyes will therefore affect final pulp brightness and whitenessAn important role of deinking chemicals in a recycling operation is to maximize ink detachment, flotation and washing while minimizing ink break-up and redeposition. On the other hand, bleaching chemicals make the pulp brighter by altering absorption properties of lignin elements and by stripping color from dyes.

In principle, the most economical pulping and flotation medium would be water, to which would be added a surfactant to assist in the detachment of ink particles during

pulping, and a flotation collector to help float ink particles. Even if ink detachment from fibres was perfect and if subsequent cleaning steps removed all detached ink, the brightness of the pulp would not equal that of the virgin fibres from which the recovered paper was originally made. One important reason is that during pulping in water alone, calcium carbonate particles used in magazine coatings bring the pH to an alkaline region where lignin moieties are darkened. In reality, ink detachment and removal by flotation and/or washing is never perfect, contributing further to brightness loss. Ih order to minimize alkaline darkening while obtaining the associated benefits of less caustic conditions, near-neutral sulphite deinking is now available commercially; however, the combination of sodium sulphite in pulper and sodium dithionite in a post-bleaching step generally leads to a slightly darker final product, when compared to the alkaline peroxide process. In mills not running 100% recycled pulp furnish in the paper machine, this brightness deficiency can be regained by better bleaching of the virgin fraction of the furnish. However, in a 100% recycling mill, only one point of brightness loss is often unacceptable.

The present invention provides an alternative way to prevent alkaline darkening originating naturally when the recovered paper contains calcium carbonate, and to bleach lignocellulosic materials present in many recovered paper grades, by employing recently discovered phosphines or phosphine precursors as bleaching agents in the deinking operations. Though this new class of bleaching chemicals has been recently proposed for treatment of virgin mechanical fibres or paper, its application to processes of recycling used fibres has not been disclosed. Moreover, it has been discovered upon usage of these new bleaching chemicals in mixtures of old newspapers (ONP) and old magazines (OMG), that specific requirements for their usage need to be met to make this new chemistry best suited for recycled pulps.

According to the present invention, alkaline darkening prevention and bleaching of lignocellulosic materials in recycled pulp can be achieved by treatment of the materials

with a water-soluble tertiary phosphine of formula (a) or a phosphonium compound of formula (b):

where R ₁ , R ₂ and R ₃ in formula (a) are groups on the tertiary phosphine selected to provide a water a solubility of at least 0.01 g/L.

In one embodiment, R ₁ , R ₂ and R ₃ are independently an alkyl group (R) or hydroxy- alkyl (HO-R ^a -) group, with R being (CH ₂ ) _q H and R ^a being preferably 1 to 6, (CH ₂ ) _q wherein q is an integer of 1 to 12; or an ether radical of formula R ₈ -O-(CH ₂ ) _P in which R ₈ is alkyl of 1 to 12, preferably 1 to 6 and p is an integer of 1 to 12, preferably 1 to 6. The groups Ri, R ₂ , R ₃ and R ₈ may be unsubstituted or substituted, and R ₁ , R ₂ and R ₃ may be the same or different.

The groups R ₁ , R ₂ , R ₃ and R ₈ may be unsubstituted or substituted. For example, in the case of R and R ^a , there may be up to (2q + 1) substituents and in the case of R ₈ -(O- CH ₂ ) _p there may be up to (4p + 1) substituents. These substituents may suitably be

selected from hydroxyl, thio, thioether, amino, ester, amide, carboxyl and carboxylate groups.

The groups R ₁ , R ₂ , R ₃ and R ₈ may be uninterrupted or interrupted by up to 6 oxygen (O) atoms, or secondary amino (NH) groups.

It will be understood that such interruptions by oxygen atoms or secondary amino must follow rules of chemistry. Thus, if there is interruption by more than one oxygen atom, the oxygen atoms will be separated by at least one methylene group. Similarly, if there is more than one secondary amino group, such secondary ^' amino groups will be separated by at least one methylene group.

Though several substitutions of groups on the phosphorus atom are possible, a preferred embodiment is a tertiary phosphine with R ₁ , R ₂ and R ₃ being identical groups. An example of such a water soluble compound is tris(hydroxymethyl)ρhosphine (THP), with R ₁ , R ₂ and R ₃ each being the group CH ₂ OH. Similarly, in formula (b), there can be a wide variety of possible groups (R ₄ , R ₅ , R ₆ , R ₇ ) in the phosphonium salt (b) containing a charged organic or inorganic anion X ^m" , such as chloride, sulfate, hydroxide, phosphate, carbonate, bicarbonate, bisulfate, alkoxide, formate, acetate, citrate, oxalate, ascorbate, ethylenediaminetetraacetate or diethylenetriaminepentaacetate wherein m is an integer of 1 to 5, preferably 1 or 2. Each group, R ₄ , R ₅ , R ₆ , and R ₇ , can be the same or different but, preferably, three or four groups are identical.

The groups R ₄ , R ₅ , R ₆ and R ₇ are selected from the same class of alkyl, hydroxyalkyl and ether radicals as R ₁ , R ₂ and R ₃ defined above, and may be unsubstituted or substituted, as for R ₁ , R ₂ and R ₃ ; R ₄ , R ₅ , R ₆ or R ₇ can also be hydrogen (H).

Examples of phospines a) are tris(hydroxymethyl) phosphine, tris(hydroxypropyl) phosphine and tris(2-carboxyethyl) phosphine.

One example of the phosphonium compound (b) is , tetrakis(hydroxymethyl)phosphonium chloride (THPC), [P(CH ₂ OH) ₄ ]Cl, commercially available (from Aldrich), and a second example of a phosphonium salt is tetrakis(hydroxymethyl)phosphonium sulphate (THPS), [P(CH ₂ OH) ₄ J ₂ SO ₄ . Conversion of phosphonium to phosphine compounds can be readily done. THP can also be synthesized from THPC in the laboratory according to a literature procedure [Ellis et al., Inorg. CJiem. Ml 3026-3033, 1992].

Diphosphines or bisphosphine and diphosphonium or bisphosphonium salts, based on (a) and (b) may also be employed. Thus, a suitable diphosphine or bisphosphine is of formula (c)

wherein R ₁ , R ₂ , R ₄ and R ₅ are as defined hereinbefore and Rg is a single bond i.e. forming a P-P linkage, or alkylene of 1 to 12, preferably 1 to 6, carbon atoms, uninterrupted or interrupted by up to 6 oxygen atoms or secondary amino (NH) groups and unsubstituted or substituted by one or more substituents of the class defined for R ₁ , R ₂ and R ₃ above.

A suitable diphosphonium or bisphosphonium salt (d) is of formula:

in which Ri, R ₂ , R ₄ and R ₅ , R ₉ and X are as defined above; t is 1 or 2; m is an integer of 1 to 5, preferably 1 or 2, and k and z are the lowest possible integers such that kt = zm; and Y ¹ and Y ² are independently selected from hydrogen, and hydroxy alkylene of 1 to 6 carbon atoms, especially hydroxymethylene (HO-CH ₂ -).

If the phosphine or phosphonium salt is added to the pulper, with or without co-addition of sodium silicate, at a near-neutral pH in the paper, extensive ink fragmentation occurs unless the soaking time is increased and/or the mixing intensity is reduced. Presence in the pulper of other additives that reduce this adverse ink break-up phenomenon in the pulper enhance the benefits of these phosphorous compounds.

Alternatively, addition of these phosphorus compounds in the late stage of pulping or after pulping offsets the ink fragmentation phenomenon induced by low pH and/or the presence of soluble phosphines and phosphonium salts.

In general, treatment of recycled pulps containing lignocellulosic materials with the phosphine or phosphonium compounds (a), (b), (c) or (d) can be carried out on recycled pulp or paper over a consistency of 0.01 to 50%, preferably 1.5 to 20%, in a pH range of 4.5-12.0, preferably 4.3 to 9.3, and a temperature range of 20 to 170 ⁰ C, preferably 20 to 130 ⁰ C, at various places during the processing of recycled pulps. To get maximum benefits associated with near-neutral conditions, the preferred pH range is 6.5 to 7.5. The usual temperature range is that of process water, which is between 40 and 6O ⁰ C but

in special cases when high brightness is required, bleaching can be carried out in special vessels at much higher temperatures, namely 80 to 120°C. Possible treatment points are in the pulper which is typically operated between 4 and 25% consistency, or the dump chest of the repulper, in which pulp consistency lies typically between 3 and 6%. Other applications can be in the disperser, in the bleaching tower or any other vessels, where the consistency typically ranges between 3 and 40%.

The amount of the phosphine or the phosphonium compound can suitably range from 0.01 to 6.0% by weight based on oven-dried pulp/fibre weight, preferably at least 0.05%, more preferably at least 0.1% and most preferably from 0.2 to 3.0%, by weight. Treatment of lignocellulosic materials with the phosphine or phosphonium compound can also be incorporated into a known, reductive bleaching such as the sodium dithionite bleaching of the lignocellulosic materials.

As described above, the phosphorous compound may be present in any one of the stages of repulping, deinking and bleaching to which the ink printed recovered paper is subjected in recycling; or the phosphorous compound may be present in more than one of the stages or in a stage intermediate these stages, for example in a holding stage between the repulping and the deinking, or between a pair of successive deinking stages.

In an especially advantageous embodiment of the invention, the phosphorous compound is employed in the repulping stage, or the deinking stage or in both the repulping and deinking stages.

In an especially preferred embodiment, the phosphorous compound is added to the pulp suspension formed in the repulping stage, conveniently with dilution of the suspension. The suspension is held for a storage time, and may be under mild mixing conditions such as results from pumping the slurry in and out of a storage tank. In this way, ink fragmentation is reduced, while the phosphorous compound bleaches the pulp. The storage or holding time may suitably be 0.1 to 4, preferably 2 to 4 hours.

Because deinking and bleaching functions are both important in the processing of old papers and because the phosphine or phosphonium compounds play no positive physicochemical role in ink detachment, redeposition and flotation, the repulping and flotation operations cannot generally be carried out with these compounds alone. Indeed, as is the case in all recycling processes, deinking is suitably assisted, for example, with nonionic, anionic or cationic surfactants. When the phosphine or phosphonium compounds are used, the choice of such surfactant is dictated by its activity at the pH of the process.

It has now been discovered that with common blends of papers, addition of water brings the pH close to 8 as a result of calcium carbonate particles present as filler or in coating formulations. Upon addition of tetrakis(hydroxymethyl)phosphonium sulphate (THPS) for example, pulp pH in the repulper falls to about 1.2-1 A and the usual combination of a nonionic surfactant of the ethylene oxide-propylene oxide block copolymer type (EO-

PO type), for example in the pulper, and of anionic fatty acid soaps ahead of flotation, works efficiently in the presence of the phosphorous compound. At lower pH, long chain calcium salts of fatty acids used as flotation aids may no longer be as effective as they progressively convert to the undissociated acid form. In this invention, repulping and flotation pH determine the type and amount of soap/surfactant required to effectively detach and float ink particles; and the phosphorous compound influences the pH.

When repulping is carried out at acidic or near-neutral conditions, penetration of water in paper is slower, which promotes ink fragmentation in the pulper. Because phosphine and phosphonium salt compounds further contribute to ink break-up, in a first embodiment, soaking time is increased and mixing conditions are made milder in the pulper in order to reduce ink fragmentation.

In another embodiment, the phosphine or phosphonium salt compounds are added after repulping, in order to circumvent ink fragmentation problems in the pulper, especially

due to low pH and the presence of the new bleaching compounds (the phosphorous compound).

In another embodiment, a surfactant effective for deinking is present with the phosphorous compound.

In a still further embodiment, the phosphorous compound is present in a range at a pH close to neutral.

In a third embodiment, addition of phosphine or phosphonium salt compounds is split between the pulper and in one or two locations following the repulping. A typical dosage in the pulper would be 0.02% to 0.2%.

In another embodiment, phosphine and phosphonium salt compounds replace commercial reducing agents such as sodium dithionite, or formamidine sulphinic acid (FAS) to post-bleach recycled pulp, whether the pulper chemistry was alkaline with hydrogen peroxide or near-neutral with sodium sulphite.

In this specification, per cents are by weight unless indicated otherwise.

EXAMPLES

A number of deinking tests were carried out on a laboratory scale. The same 70/30 mixtures of old newspaper (ONP) and magazines (OMG) was used for all of the following examples. The OMG fraction consisted of the same four pages of a single light weight coated (LWC) paper printed by the heatset offset process whereas the same issue of the ONP fraction was used. Newspaper had been printed by coldset offset process. When received from the printer, the two paper sources were aged at room temperature for one month and then stored at 5°C until used.

Repulping of paper mixture was carried out at 10% consistency (on an o.d. paper basis) in a helical pulper equipped with variable speed drive to change rotational speed. An

amount of 0.625 kg of paper was added to the deinking solution which had previously been warmed up to 50 ⁰ C.

When the conventional alkaline chemistry was used in the pulper the following chemicals were mixed with the paper charge at these dosages: 0.8% NaOH, 2% Na ₂ SiO ₃

(37° Beaume), 0.06% EO/PO nonionic surfactant, 0.6% sodium oleate and 1% H ₂ O ₂ (all

%'s, on an o.d. paper basis). The sulphite near-neutral chemistry in pulper used here employed 0.06% nonionic surfactant Resolution 4283, an ethylene oxide-propylene oxide block copolymer from Eka Chemicals, 1.5% sodium sulphite, 1% sodium silicate and 0.4% emulsified fatty acids. Unless otherwise specified, pulping with commercially available THPS (Aldrich) was done at a dosage of 1.0% using Eka 4283 surfactant

(0.06%) which was also used for the sulphite deinking pulping.

After a predetermined pulping time, an aliquot of pulp diluted to 4% with tap water, was sealed in a polyethylene bag that was held at constant temperature for variables times in a holding tank, similar to a dump chest in a deinking mill. Pulp pads (170 g/m ² grammage) were prepared from samples taken in the pulper and in the holding tank, before and after hyperwashing. This hyperwashing treatment consists in washing the pulp until all ink detached from the fiber surface is removed from the pulp slurry.

Reflectance was measured at 457 run and 950 nm, respectively, using a Technibrite TB- IC (trade-mark). These measurements allowed the determination of % ISO brightness and of Effective Residual Ink Concentration (ERIC, in ppm), which is now a standard measurement for recycled pulp (TAPPI 567 pm-97). From ERIC measurements made before and after hyperwashing, it was possible to calculate total, free and bound ink concentrations. Bound ink represents the content of ink still attached to long fibres after a processing step.

EXAMPLE 1

Table 1. % ISO and ERIC of unwashed and washed pulp sampled in pulper; comparison between conventional alkaline, sulphite and THPS pulper chemistries at fixed impeller speed of 800 rpm and pulping time of 3 min.

After a pulping time of 3 minutes, sulphite and THPS treated pulps had brightness lower than alkaline peroxide treated pulp. Degree of ink fragmentation was greater with sulphite and THPS treated pulps than with the alkaline treated pulp, as indicated by the higher ERIC values after 3 min of pulping. Low ERIC values after hyperwashing indicate that in all three cases, ink detachment was good, although the alkaline peroxide chemistry in the pulper was superior to the two near-neutral chemistries.

EXAMPLE II

Table 2. % ISO and ERIC of washed pulp sampled after allowing additional storage time after THPS repulping; pulping time: 3 min; pulping mixing speed: 400 rpm; storage at 50 ⁰ C without mixing; THPS dosage: 1%).

When THPS treated pulp in pulper was diluted to 4% consistency and then held at 50 ⁰ C in plastic bags to permit bleaching to continue, brightness improved significantly. Indeed, the brightness obtainable after 2 hours reached that of the alkaline peroxide treated pulp (in Example I). This demonstrates that THPS continues to be active for 3-4 hours after the short pulping of 3 min.

EXAMPLE in

Table 3. % ISO and ERIC of unwashed and washed pulp sampled in pulper; mixing speed during repulping was varied from 200 to 800 rpm (pulping time: 6 min; THPS dosage: 1%). ,

When rotational speed of helical impeller exceeded 400 rpm, ink fragmented to a greater extent as indicated by the increase in ERIC values after 6 min pulping. Despite greater ink break-up at 800 rpm, brightness after hyperwashing was greater, thus indicating that more intense mixing accelerated reductive bleaching of fibres by THPS.

EXAMPLE IV

Table 4 % ISO and ERIC of unwashed and washed pulp sampled in pulper; pulping time during THPS pulping was changed from 3 to 6 min (pulping mixing speed: 400 rpm; THPS dosage: 1%).

As pulping time increases at a fixed rpm of 400 rpm, ERIC after pulping increased from 1240 to 1473 ppm in going from 3 to 6 min pulping time, thus indicating that the ink particles in THPS- treated pulps are prone to fragment as a result of mixing.

EXAMPLE V

Table 5. % ISO and ERIC of unwashed and washed pulp sampled in pulper when sodium silicate during THPS pulping was raised from 0 to 2% (pulping time: 2 min; pulping mixing speed: 800 rpm; THPS dosage: 1%).

In this example, sodium silicate, an ingredient of alkaline and sulphite deinking chemistries was added along with THPS, at a dosage of 1%. In the pulping step at least, the usually beneficial effect of sodium silicate for prevention of ink break-up and redeposition is not very significant when the THPS chemistry is added in the pulper.

EXAMPLE VI Table 6. % ISO and ERIC of unwashed and washed pulp sampled in pulper when THPS dosage was raised from 0 to 2% (pulping time: 2 min; pulping mixing speed: 800 rpm; silicate: 2%).

Increasing THPS dosage during a short pulping of 2 min raises the degree of ink fragmentation, as indicated by the significant increase in ERIC after pulping. The rise in ERIC values on washed pulps when THPS dosage increases, is an indication that the fragmented ink particles are prone to redeposit in the fibre lumen. When THPS is added during pulping, a high dosage is not beneficial because the short pulping time is not long enough to get the full bleaching benefit of a greater dosage, while the enhanced ink fragmentation and redeposition phenomena predominate over the bleaching effect. Adding THPS after pulping may then be a better strategy.

EXAMPLE VII

Table 7. % ISO and ERIC of unwashed and washed pulp sampled in pulper when water hardness was increased from 0 to 150 ppm. (THPS: 2%; silicate: 2%; pulping time: 2 min; pulping mixing speed: 800 rpm).

In this test series, THPS was added in the pulper along with sodium silicate and calcium chloride. Under these conditions, increasing water hardness had no effect on ink fragmentation or on brightness of unwashed and washed pulps taken from pulper after a time of 2 min.

EXAMPLE Vπi

Table 8. ERIC of unwashed pulp and % ISO of washed pulp sampled after repulping in water alone and then adding THPS to pulp diluted to 4% consistency; pulping time: 3 min; pulping mixing speed: 400 rpm; storage at 50 ⁰ C without mixing; THPS dosage:

In this example, the blend of papers was repulped in water alone. After repulping, pulp was diluted with water, from 10 to 4% consistency after which, THPS was added. The diluted pulp containing THPS was then held at 5O ⁰ C. During the holding time, the brightness improved significantly. Though absence of THPS and surfactant during repulping was found to reduce ink fragmentation (ERIC decreased from 1350 to 1231 ppm, with and without THPS), brightness of hyperwashed pulp was lower because attached ink increased from 89 to 125 ppm when only water was used in the repulping.