Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F1/00—Wet end of machines for making continuous webs of paper
  • D21F1/66—Pulp catching, de-watering, or recovering; Re-use of pulp-water
• • 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
  • D21H17/01—Waste products, e.g. sludge

Definitions

• the present invention relates to the treating of circulation waters in papermaking.
• Calcium carbonate CaCO 3 is commonly used in paper making as a filler or pigment because it has a high brightness. It is also very white and can be purchased at reasonable prices.
• the calcium carbonate may be naturally occurring chalk or calcite or it may be synthetically produced precipitated calcium carbonate (PCC). It is well known in the art that the calcium carbonate dissolves in acids and the more acid is present the better the dissolution is. Calcium carbonate is sparingly soluble in alkaline conditions above a pH of about 8, but it is attacked by acids such as sulfuric acid and alum, as a result of which it is solubilized. According to literature ( D. Eklund, T. Lindström, Paper Chemistry - an introduction, DT Paper Science Publications, Grankulla, Finland 1991, p. 253 ) an increase in the carbon dioxide partial pressure increases the solubility of calcium carbonate. This can be presented with the following equation: (1) CaCO 3 + CO 2 + H 2 O ⁇ Ca(HCO 3 ) 2
• a part of the calcium carbonate used as a filler in the paper machine dissolves into the process waters because of for example the pH gradients between the mass and the circulation waters, acidic additive chemicals or acids caused by microbiological activity. Some calcium is also provided with the wood. Dissolved calcium reacts for example with extracts causing harmful precipitates.
• Recycled waste paper as well as broke may contain calcium carbonate as filler and/or pigment.
• the repulping of recycled fibers is generally performed at an alkaline pH wherein the calcium carbonate remains essentially in solid form. However, if the paper machine is run at an acidic, neutral or pseudoneutral pH range, the calcium carbonate deriving from the recycled fibers will start to dissolve.
• Carbon dioxide is a gas, which easily dissolves in water or a pulp suspension forming carbonic acid and/or bicarbonate ions according to the reaction: (5) CO 2 + H 2 O ⁇ H 2 CO 3 ⁇ H + + HCO 3 -
• WO99/00545 describes a method of reducing the content of calcium in a pulp suspension by alkalizing the pulp suspension to a pH above 8 and adding carbonate ions to the pulp suspension in an amount of at least 0.1 kg carbonate ions by the ton pulp for formation of calcium compounds which are adsorbed on the pulp. Calcium is transferred from the liquid phase to the pulp fibers.
• the consistency lies suitably within the interval from 8 to 16 %.
• the invention does not relate to circulation waters.
• WO 9945192 discloses a method for treating process water in a bleaching plant. Calcium and transition metals are removed by addition of magnesium, a carbonate source, white liquor and flocculant.
• the carbonate source may be carbon dioxide.
• calcium hardness of recycled paper-processing water is controlled by precipitating calcium ion in process water into calcium carbonate through addition of sodium carbonate or sodium hydrogen carbonate to white water short-circulated in a papermaking process and removing the calcium carbonate precipitate from the water.
• Air or carbon dioxide may be used for removal of the precipitate by flotation.
• US 3,639,206 describes removal of ligneous colour bodies and suspended fibrous material, whereby waste water effluent is treated first with a calcium salt (CaO, Ca(OH) 2 ). The separated aqueous phase is then contacted with CO 2 gas to precipitate any dissolved calcium salts and a final separation is performed.
• a calcium salt CaO, Ca(OH) 2
• US 5,679,220 discloses a method enhancing the deposition and retention of particulate filler on papermaking fibers wherein separate slurries of fibers and Ca(OH) 2 or equivalent thereof are combined with gaseous precipitant (CO 2 ) to form in situ calcium carbonate on the fibers.
• gaseous precipitant CO 2
• carbon dioxide may be used for increasing the solubility of calcium carbonate and for the hardening of recycled waters used in the treatment of pulp from waste paper.
• precipitation of gypsum in an alkaline recycle or broke derived pulp may be prevented by adding carbon dioxide to form bicarbonate ions in the alkaline pulp and to precipitate calcium as carbonate.
• WO 98/56988 the pH of a pulp suspension in the stock preparation of a paper machine is stabilized by carbon dioxide.
• a combination of sodium hydroxide and carbon dioxide provide a significant buffering effect in the pulp.
• the stabilized pH reduces the dissolution of calcium carbonate at acidic addition points.
• Closed circulating water systems are used more and more in order to lower the amount of water discharged from the papermaking process.
• the method according to the present invention enables in situ cleaning of the circulation waters.
• the method is defined in the appended claims, the contents of which form a part of this specification.
• the present invention relates to a method for the treating of process water circulating in a process for producing paper.
• the method comprises the steps of providing an aqueous cellulosic pulp suspension in a papermaking process, said suspension containing calcium carbonate added as filler, causing free calcium ions to be present in said aqueous cellulosic pulp suspension due to solubilization of said calcium carbonate, - in a first unit operation of said papermaking process, separating a stream of process water from said aqueous cellulosic pulp suspension, said separated process water having an elevated level of free calcium ions, - adding sodium hydroxide to said process water for providing therein, without the addition of calcium oxide or hydroxide, a pH of not less than pH 9, subsequently distributing gaseous carbon dioxide in said process water in an amount sufficient for lowering the pH of said water to a pH level of 8.5 to 6.0 for precipitating a substantial part of said calcium ions as solid calcium carbonate in said water and for obtaining a decreased level of free calcium ions
• the present invention provides a cleaning station, i.e. a kind of kidney in the circulating process waters.
• the kidney removes the excess calcium ions and solves the problems caused by the elevated levels of calcium ions in the papermaking process. It enables the paper maker to continuously circulate the same waters without disturbing calcium precipitations.
• Sodium bicarbonate has also been used in the prior art in order to remove calcium.
• the addition of sodium bicarbonate e.g. as a slurry may be complicated and the pH will adjust itself only to about 8.
• the addition of carbon dioxide as disclosed by the present invention can be performed more flexibly into the process in a desired part of the process.
• process waters are separated from the pulp suspension and cleaned from calcium ions separately from the pulp suspension.
• the separation takes place in one unit operation and the calcium ion depleted waters are reintroduced into the process at another unit operation in the system.
• the stream of process water to be treated may comprise the whole or part of the waters separated from the pulp suspension in the first unit operation.
• a typical first unit operation of the invention is the dewatering of a pulp suspension in the web forming section of a paper machine.
• the water drained through the wire contains elevated amounts of calcium ions. If these calcium ions are not removed from the water, the calcium ion level increases for each loop that the water makes.
• the circulation or loop that the separated water makes is generally called short circulation or long circulation.
• the present invention is applicable to both types of process waters.
• the short circulation water is used in a second unit operation which comprises the dilution of the stock just before it is made into paper.
• the long circulation water may be used in various unit operations upstream in the process for purposes which are well known to those skilled in the art.
• Calcium ion containing waters may also be found in other unit operations or clear filtrates and the calcium ion depleted water may be returned to the same or to another unit operation in the papermaking process.
• the "elevated level" of calcium ions as used in the present specification and claims is intended to mean any amount of calcium ions which is detrimental to the process.
• the acceptable level of calcium ions will vary greatly with the process as those skilled in the art know. For instance in the short circulation waters levels of about 100 mg Ca++/l may be acceptable, while severe problems may be caused if the calcium ion concentration rises above 150 mg/l or 200 mg/l.
• the extent to which the calcium ions are removed from the process waters depends on the pH of the treated water.
• the process water which is provided in the process and which is treated by the steps of the process will initially have a pH of not less than 9 and the kidney may function satisfactorily at such a pH.
• a higher pH such as a pH above 10, or most preferably above 11.
• Calcium carbonate is precipitated more readily the higher the pH is. Therefore, in a preferred embodiment of the invention a pH which is higher than pH 9, and which is preferably 10 to 11 is provided in the process water.
• the desired pH of the process water may be provided by adding an alkaline compound such as sodium or potassium hydroxide. It is not in accordance with the concept of the present invention to raise the pH with lime, i.e. calcium oxide or calcium hydroxide since these compounds add to the calcium ion content of the water. In the present invention the intention is to remove the calcium ions existing in the separated process waters as a precipitate. The idea is not, as in many prior art operations, to obtain as much solid calcium carbonate as possible.
• the gaseous carbon dioxide which is used according to the invention is a gas which contains a substantial amount of carbon dioxide such that it will effectively achieve the object of precipitating calcium carbonate in the environment.
• the gas preferably contains at least 20 %, preferably more than 50 %, most preferably more than 90 % carbon dioxide.
• the gas may include other components which are not detrimental to the purpose of the gas in the invention and it may be, for instance flue gas containing carbon dioxide and sulfur dioxide.
• the separated process waters may contain small amounts of solid calcium carbonate.
• Such solid calcium carbonate may improve the operation of the kidney in that the small solid particles act as seeds or growth centers for the calcium carbonate precipitating from the liquid. It may, in fact, be useful to add a small amount of finely ground solid calcium carbonate to the process water after the separation step to ascertain that there are sufficient amounts of seeds in the solution. Such additional calcium carbonate should not be added at a pH lower than 8.
• an alkaline compound as well as the addition of carbon dioxide is monitored mainly by pH.
• carbon dioxide or a carbon dioxide containing gas would generally be used to lower the pH to about 9 or below.
• the pH of the water is a pH level of 8.5 to 6.0.
• the pH is lowered to pH 8.0 to 6.5, most preferably pH 7.5 to 7.0.
• Another acid than carbon dioxide may be used for lowering the pH below 9, but the use of carbon dioxide is preferred. Strong acids such as sulfuric acid may dissolve the precipitated calcium carbonate and should thus be avoided at least at pH below 8.
• the proper functioning of the precipitation may be monitored by measuring the amount of free calcium ions in the in and out going waters.
• a substantial part of the calcium ions should be removed in the form of calcium carbonate so that an increased accumulation of calcium ions can be avoided. It is not possible to remove all calcium ions from the solution but a person skilled in the art will know the degree of calcium ion concentration acceptable in the process in question and will be able e.g. by raising the pH to a higher level and by subsequently adding more carbon dioxide to increase the precipitation and to provide a desired depletion of calcium ion from the water.
• an alkaline compound is introduced first, for instance in a pipe or in a small mixing tank. Thereafter the carbon dioxide is added.
• the addition of carbon dioxide generally requires a pressurized addition point and therefore a suitable point is after a pump in a pipe line or in a pressurized tank or reactor.
• the precipitation of calcium carbonate starts immediately when the carbon dioxide contacts the calcium ion containing alkaline liquid.
• the precipitated calcium carbonate may be removed from the water by conventional means such as by filtration or it may be left in the water for acting as a precipitated filler in the subsequent paper making.
• the treated water has a decreased concentration of calcium ions and when it is subsequently returned to the process at a second unit operation, such as dilution, washing etc. there will be no problems with calcium precipitation etc.
• Calcium carbonate was added as filler and the white water contained dissolved calcium carbonate which gradually caused the calcium ion content to accumulate to elevated levels (over 200 mg Ca 2+ /l). Precipitations of calcium salts and sticky particles occured in the process.
• a kidney was installed in the white water circulation system.
• the pH of the circulation water was raised to a level of about pH 11 with NaOH.
• CO 2 was added to the water to cause precipitation of the calcium as calcium carbonate.
• the amount of dissolved calcium decreased from a level of 210 mg Ca 2+ /l to a level of 110 mg Ca 2+ /l.
• CO 2 was added until a pH of about 8 had been reached.
• the water was returned to the process at this pH.
• the formed calcium carbonate followed the paper produced in the paper machine out from the process.
• a paper mill uses a pulp suspension consisting of a blend of de-inked pulp (DIP) and thermomechanical pulp (TMP).
• DIP de-inked pulp
• TMP thermomechanical pulp
• the pulp contains a significant amount of calcium carbonate deriving from the waste paper.
• the pH of the stock is adjusted to pH 6.8 to 6.9 with sulfuric acid and the paper machine is run at this pH.
• a kidney in accordance with the invention is adopted in the system and the short circulation waters are treated first by raising the pH to 10 with sodium hydroxide and then by adding carbon dioxide to the water until the pH has decreased to 8. A part of the precipitated calcium carbonate is removed by sedimentation in a tank. Additional carbon dioxide is then added to the water to bring the pH back to 6.8 to 6.9.

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• Paper (AREA)
• Chemical Treatment Of Metals (AREA)
• Separation Using Semi-Permeable Membranes (AREA)
• Removal Of Specific Substances (AREA)

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• 2001
  • 2001-05-30 FI FI20011131A patent/FI20011131A/fi not_active Application Discontinuation
• 2002
  • 2002-05-30 AT AT02774036T patent/ATE423867T1/de not_active IP Right Cessation
  • 2002-05-30 WO PCT/FI2002/000463 patent/WO2002097189A1/en not_active Application Discontinuation
  • 2002-05-30 DE DE60231303T patent/DE60231303D1/de not_active Expired - Fee Related
  • 2002-05-30 EP EP02774036A patent/EP1402111B1/en not_active Expired - Lifetime

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