Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
  • D21H23/06—Controlling the addition
  • D21H23/08—Controlling the addition by measuring pulp properties, e.g. zeta potential, pH
  • D21H23/10—Controlling the addition by measuring pulp properties, e.g. zeta potential, pH at least two kinds of compounds being added
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/63—Inorganic compounds
  • D21H17/65—Acid compounds

Definitions

• the present invention relates to an improvement to paper manufacturing processes.
• It relates more particularly to a process for manufacturing a paper product from paper pulp containing cellulose fibres dispersed in an aqueous medium, which pulp may come from various sources.
• paper and paper product are employed without distinction in the description. Without being restrictive, they may be: paper for graphical use, packaging paper, domestic paper, various special papers, together with board and the like, including also intermediate products of the dried cellulose pulp type.
• the manufacture of paper products generally comprises several successive steps: firstly, the paper pulp is manufactured from various fibrous materials containing cellulose (wood and/or annual plants) , water and chemical agents.
• This paper pulp - a suspension of fibres in water - is the starting product for further steps during which it is treated.
• the fibres dispersed in water are worked in order to obtain the desired paper characteristics of the final product - in particular, they are bleached, refined, washed and entangled. They are then dewatered 'and dried. During dewatering, the fibres have the property of naturally adhering to one another, thus forming a fibrous mat.
• various non-fibrous materials such as fillers, dyes, starches and other auxiliary products.
• these various additives may be added to the fibre suspension before dewatering - they will thus be present in the fibrous mat; they may also be added by depositing them on the surface of the web of paper after the dewatering step.
• This incorporation of chemical additives to the fibrous mat and/or to the web has the purpose of giving the final product particular properties specific to its use; mention may be made of sizing agents, dewatering and retention agents, starches, fillers, dyes, antifoams, wet-strength or WS agents, etc.
• the invention applies broadly to the various processes for manufacturing paper products, and especially to those in which one or more of the aqueous vehicles is completely or partially recycled within the papermaking plant where they circulate in closed or semi-closed systems, as well as to those in which broke is incorporated.
• broke refers to the paper waste arising from the process, especially that recovered during formation of the web of paper, for example coming from web breaks or web edge trimmings; papers from various sources, coated or uncoated papers coming from one or more paper machines, and paper waste from various sources may thus be found therein.
• the broke is treated in a particular system called the broke system.
• semi-closed (or semi-open) system refers to any system in which a portion of the circulating aqueous vehicle is recycled; the term “closed system” refers to any system in which all of the aqueous vehicle flowing therein is recycled - only make-up water is added in order to compensate for evaporation.
• aqueous vehicle itself denotes any aqueous medium and especially the dilution water, cellulose stock when its water content is such that it behaves as a fluid (consistency of around 15% of fibres by weight or less) , effluents arising from the papermaking process, or any other aqueous fluid circulating in its systems .
• This white water conventionally represents at least • 80% and preferably at least 90% by volume of the dilution water; the balance consists especially of other process water and/or fresh water.
• the terms "white water” and "dilution water” are used without distinction; the context allows a person skilled in the art to know, without any ambiguity, the meaning to be ascribed to this term when a distinction proves to be necessary.
• the zeta potential expressed in millivolts (mV) , is an electrical characteristic of solid particles, fibres or fillers, present in the water of the paper stock; it is connected to the distribution of electrical charges on the surface of the particles in question.
• the zeta potential of the fibres is subject to variations due to the presence of the various ion-generating chemicals which, when added during the paper manufacture, have an influence on the distribution of charges present on their surface.
• the zeta potential has an indubitable influence on the retention and drainage mechanisms, and therefore on the formation of the paper web, another parameter is also important, namely the ionic demand.
• the latter characterizes the charge on the particles present in solution. It is measured by titration using an electrolyte. A specific amount of a filtrate is neutralized by an electrolyte; at the neutralization point, the amount of electrolyte used represents the value of the ionic demand.
• the cationic demand is -5 ml.
• Patent WO 99/54741 It is known from Patent WO 99/54741 to continuously monitor the formation of the web of paper by measuring the zeta potential of the fibres of a fibrous dispersion and by adjusting the amount of chemical additives added depending on its measured value. More specifically, the process parameters of the paper machine are adjusted, in a first stage, so as to obtain a paper of the required quality. The zeta potential is then measured at various points in the process . The zeta potential profile thus determined is called the optimum zeta potential profile. During continuous production, the objective is therefore to maintain this optimum zeta potential profile. When the potential profile departs from this optimum profile, the process parameters, and especially the amount of chemical additives, are modified so as to approach the optimum profile as was determined.
• the object of the present invention is to act on the charged sites present in the stock, upstream of the points of introduction of the additives, by carrying out a pre-treatment of the aqueous medium by means of carbon dioxide .
• the object of the present invention is thus 1) to reduce the amount of additives to be initially added and 2) to provide a means of acting on the zeta potential other than the a posteriori addition of chemical additives.
• the invention consists in acting preventatively on the elements that are disturbing the overall electrochemical system of the assembly formed by the aqueous medium and the fibres, in such a way that the additives are fixed to the fibres instead of interacting with the ionic compounds present in the aqueous medium.
• the invention also consists in controlling the ionic demand of the medium.
• the aim of the invention is to supply the stream of stock with carbon dioxide, and to do so prior to incorporation of the additives.
• This supply of carbon dioxide may be accomplished by direct injection into the stream of stock, or by indirect injection via an aqueous vehicle that will feed into this stream.
• the carbon dioxide thus added modifies the physico-chemical behaviour of the fibre-filler/aqueous medium assembly.
• carbon dioxide is injected into the paper stock upstream of the points of addition of the additives, a change in the ionic demand with respect to that in the absence of carbon dioxide is induced.
• the medium behaves as if the amount of anionic trash available were reduced, thereby allowing the cationic additives to be preferentially fixed to the fibre during their incorporation .
• the process according to the invention is a process for manufacturing a paper product from a paper stock containing cellulose fibres dispersed in an aqueous medium, which process comprises: at least one step of adding chemical additives; - a step of forming the paper product from the stock by mechanical separation of the fibres from the aqueous medium in order to form a fibrous mat; and at least one step of injecting carbon dioxide into the paper stock, characterized in that at least carbon dioxide is injected into the stock upstream of at least one step of adding chemical additives so as to control the ionic demand of the aqueous medium downstream of the carbon dioxide injection point.
• the additives generally cationic additives
• they are not fixed, or are fixed in fewer numbers, to the fibres because the anionic trash interferes with their action - there is competition between the anionic sites of the fibres and of the medium.
• the carbon dioxide injected upstream of the point of addition of the additives acts on the anionic trash, not only at the point of carbon dioxide injection but also in all the communicating systems. This is manifested by a less negative and more stable cationic demand of the system when the situation with and without addition of carbon dioxide is compared.
• the injection of carbon dioxide upstream of a step of adding chemical additives furthermore makes it possible to stabilize the zeta potential of the fibres at the step of adding the chemical additives.
• the carbon dioxide is injected in sufficient amount to control the cationic demand, it is observed that until the ions in solution have reacted the measured zeta potential changes little. After the anionic trash has reacted, the auxiliaries act on the fibre - the zeta potential then changes.
• the carbon dioxide may be injected at various points in the papermaking system, most particularly when mixtures of pulps are produced.
• the stock stream(s) flowing in the paper manufacturing system is (are) at least partly supplied with aqueous vehicles circulating in the closed or semi-open systems forming part of the paper manufacturing system and carbon dioxide is injected into at least one aqueous vehicle feeding into at least one stream of stock.
• the process includes at least one step of diluting the pulp with dilution water containing white water removed from the fibrous mat during the step of forming the paper product and carbon dioxide is injected into the dilution water upstream of the dilution step.
• chemical additives are added to the stock in the mixing chest upstream of the short system and carbon dioxide is injected into at least one stream of stock feeding into the mixing chest.
• the pulp obtained from broke enters the composition of the paper stock and carbon dioxide is injected into the broke system.
• carbon dioxide is injected into the dilution water on the output side of the broke pulper and before refining.
• At least some of the carbon dioxide is injected into the dilution water used to dilute the pulp in the short system of the papermaking plant and/or directly into the diluted stock upstream of the paper machine.
• the stock entering the machine is treated as is the white water leaving the machine and the streams of stock into which it will be reincorporated.
• At least some of the carbon dioxide is injected into an effluent of the plant, to be treated, prior to the treatment, and, after the treatment, the effluent is reintroduced, at least partly, into a system of the paper manufacturing plant.
• This treated effluent part reintroduced into the plant is generally in the form of dilution water, but it may also contain fibres recovered during treatment of the effluent and reincorporated into a stock stream.
• the process is carried out by slaving the carbon dioxide injection to the measurement of the zeta potential value and/or the ionic demand value. This measurement is preferably carried out continuously on the diluted stock.
• the carbon dioxide may be injected in liquid form.
• It may be injected in gaseous form, especially in the case of carbon dioxide injection into the short system.
• It may be injected partially in liquid form and partially in gaseous form.
• the amount of carbon dioxide injected may be from 0.5 to 15 kg of carbon dioxide per metric ton of paper product and preferably from 0.5 to 3 kg of carbon dioxide per metric ton of paper product .
• Figure 1 indicates certain essential operations of the process for manufacturing a web of paper.
• the pulp is refined in the refiner 1 and introduced into the mixing chest 2 , in which various additional materials, including cationic additives and broke, if there is any, are also introduced at 3.
• This short system conventionally, but not necessarily, comprises a purification step making use of one of the following devices or of equivalent devices: cyclone purification devices 6, generally of the cyclone washer type; devices 7 intended for removing the gas contained in the dilute stock in order to improve the homogeneity of the stock introduced into the head box of the paper machine, so as not to disturb the formation of the web; and a device 8 intended to carry out an additional purification step for the purpose of removing the last particles.
• This device is generally referred to as a "screen" .
• the head element 11 of the paper machine delivers a ply of fibrous suspension identical over the width of a forming fabric.
• the free water and the fibres are then mechanically separated, the stock is dewatered and the web of paper is then formed by the deposition of the fibres, thus constituting the fibrous mat.
• the free water drained away through the forming fabric by gravity and optionally with the aid of vacuum pumps, forms the white water. This is used in particular in the short system and in the broke system, but it is also transported via lines (not shown) of the secondary system to other semi-open systems of the papermaking plant .
• the web On leaving the head box, the web penetrates the section 12 of the paper machine, called the press section, where the web is dried to a dryness of around 40%. It then leaves the wet end of the machine and enters the drying section 13 of the paper machine where the remaining water is removed by evaporation. At this point, the web reaches a dryness of greater than 90%, preferably greater than 95%.
• the surface of the web is then treated by smoothing and calendering.
• various elements in particular fine pigments and adhesives contained in a composition called a coating slip, are deposited on the web.
• this coating operation is only carried out for certain paper applications, for example for the manufacture of papers intended for writing or printing. It is not necessarily carried out on the paper machine itself - it may be done away from the machine, after the smoothing operations .
• the trial was carried out on bleached virgin pulp containing long and short fibres. Fillers consisting of natural calcium carbonate were added for the purpose of increasing the whiteness and the opacity of the paper.
• Carbon dioxide was injected into the white water system 9 of the short system at the delivery pump of the white water recovery chest (not indicated in Figure 1) ; the amount of C0 introduced was 2.5 kg per metric ton of paper produced.
• the cationic demand was measured (the measurement being taken on the filtrate by a Mutek apparatus of the "PCD" type) at various points without addition of carbon dioxide and with addition of carbon dioxide under the conditions indicated below. The results are expressed in ml .
• the following trial corresponded to virgin pulp composed of a mixture of short fibres and . long fibres in a proportion of 50/50 into which WS (wet strength) type broke was incorporated. Carbon dioxide was injected into the stream of broke pulp before the mixing chest 2 and then also into the dilute stock in the short system before the head box 11.
• the stream of broke represents approximately 20% of the total stream of stock.
• the amounts of carbon dioxide were 5 and 3 kg per metric tonne of dry pulp, respectively.
• the additions of cationic polymers of the WS agent type and starches are carried out in the .mixing chest and at the mixing pump before the head box.
• the cationic demand (the measurement being made on the filtrate by a "PCD” apparatus and the result expressed in ml) without addition of carbon dioxide and with addition of carbon dioxide under the conditions indicated above;
• the zeta potential of the cellulose fibre (the measurement being carried out by a "zetameter” type measurement apparatus and the result expressed in mV) .
• the saving in auxiliaries of the cationic starch, dye and sizing agent type is around 15 to 20% depending on the type of pulp used and the type of paper produced.
• the fixing of the fillers added to the cellulose pulp is improved by 5%.

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• 2002
  • 2002-03-07 FR FR0202880A patent/FR2836938B1/fr not_active Expired - Fee Related
• 2003
  • 2003-03-03 AU AU2003214490A patent/AU2003214490A1/en not_active Abandoned
  • 2003-03-03 CN CN 03805406 patent/CN1639420A/zh active Pending
  • 2003-03-03 CA CA002477950A patent/CA2477950A1/en not_active Abandoned
  • 2003-03-03 EP EP03710066A patent/EP1490551A2/de not_active Withdrawn
  • 2003-03-03 WO PCT/IB2003/000818 patent/WO2003074788A2/en not_active Ceased
  • 2003-03-06 AR ARP030100763 patent/AR038884A1/es unknown

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