BLEACHING OF CHEMICAL PULP

Abstract: A method of bleaching chemical pulp in a sequence comprising at least four bleaching steps with final bleaching in a first and a second chlorine dioxide step. Between the chlorine dioxide steps an alkaline extraction is carried out, and washing takes place between the first chlorine dioxide step and extraction. Immediately after said washing step, NaOH is charged in an amount of 4-10 kg/ton pulp. Thereafter an oxidizing agent is admixed in an amount of at maximum 2 kg/ton pulp. After 15-120 seconds an acid is added for terminating the oxidation treatment and for lowering the pH-value, but without effecting a complete neutralization of residual alkaline.

Description: Bleaching of chemical pulp This invention relates to bleaching of chemical pulp in a bleaching sequence comprising at least four bleaching steps, where the final bleaching is carried out in a first and a second chlorine dioxide step with an intermediate alkaline extraction. This type of multi-step bleaching can comprise a first step with chlorine/chlorine dioxide (. CO ) , a second step with alkaline extraction where oxygen gas is added <EO) , and final bleaching in two chlorine dioxide steps <D1 and, respectively, D2). Also other bleaching sequences may occur, which are finalized with two chlorine dioxide steps with or without an intermediate extraction. It is known, that the alkaline extraction between the chlorine dioxide steps <D1, D2> at the final bleaching of chemical pulp can be simplified, for example, by abandoning washing of the pulp prior to the alkaline treatment, or by abandoning washing of the pulp after the alkaline treatment. It was then thought necessary to neutralize residual alkaline prior to the second chlorine dioxide step (D2). This implies, however, a greater consumption of alkaline or chlorine dioxide, because the pH-value in the second chlorine dioxide step <D2) will not be at optimum. This invention solves this problem and brings about a sur¬ prising and substantial increase in efficiency of the alkali- zation, which results in improved bleaching compared with conventional methods according to above. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the following, with reference to an embodiment thereof, the results of which are shown in Figure 1, which shows the brightness of the pulp expressed in % ISO as a function of the total consumption of chlorine dioxide (CIO2) in the chlorine dioxide steps expressed in kg active ehj-&r,gie per ton pulp (kg act Cl/ton) . The pulp is bleached in a conventional way in a first chlorine/chlorine dioxide step, a second alkaline extraction step reinforced with oxygen gas, and a first chlorine dioxide step. Immediately after the washing subsequent to this first chlorine dioxide step 4-10, preferably 4-8 kg NaOH per ton pulp is charged. Directly thereafter an oxidizing agent in the form of oxygen gas and/or hydrogen peroxide in an amount of at maximum 2 kg/ton pulp is admixed, each by means of a pump intended for this purpose or a mixer of a kind known per se. At mere hydrogen peroxide charging the amount can be limited to 1 kg/ton pulp. The admixing is carried out at a pressure of 0,1-1,0 MPa. After a period of 15-120 seconds, preferably 15-90 seconds, counted from the addition of oxygen gas and/or hydrogen peroxide, an acid is added. During this time a temperature of 60-90°C, preferably 60-75°C, is maintained. The acid, for example sulphuric acid CH2SO4), is added in order to terminate the treatment with oxygen gas and/or hydrogen peroxide and in order to lower the pH-value, ' but without effecting a complete neutralization of the residual alkaline. This implies, that the amount of acid per ton pulp expressed as hydrogen ions shall amount at maximum to H + = NaOH - 2 kmol H + /ton pulp 40 where NaOH is the charged amount of alkaline expressed in kg NaOH per ton pulp. The addition of acid should be made immediately before or together with the chlorine dioxide addition in the second chlorine dioxide step CD2). The Figure shows how the brightness varies with the total consumption of CIO2 at the Dl- and D2-steps when coniferous wood sulphate pulp with Kappanumber 18 is bleached. In addition to the curve referring to the invention, D<EOPD) , curves have been drawn for comparison which show the result of conventional bleaching methods. DD refers to a bleaching sequence where both final chlorine dioxide steps are carried out with intermediate washing, but without intermediate alkaline extraction. DED refers to a bleaching sequence where a conventional alkaline extraction is carried out between the chlorine dioxide steps, and washing is carried out both before and after the extraction. D(ED) refers to a bleaching sequence where no washing is carried out between the extraction and second chlorine dioxide step. The bleaching conditions in all other respects have been equivalent. The first bleaching step was carried out with a combination of chlorine and chlorine dioxide, and the second as an alkaline extraction step reinforced with oxygen gas. In the Figure, the dashed area represents the improvement, which can be obtained with the method according to the invention, compared with known art. It appears, thus, from the Figure, that according to the invention a certain brightness can be achieved by consuming considerably less chemicals. Alternatively, a higher brightness can be achieved with a certain chemical consumption. The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the invention idea .

Claims: Claims 1. A method of bleaching chemical pulp in a sequence comprising at least four bleaching steps, which sequence comprises final bleaching in a first and a second chlorine dioxide step with intermediate alkaline extraction, where washing of the pulp takes place between the first chlorine dioxide step and extraction, characterized in a) that NaOH in an amount of 4-10 kg/ton pulp is charged immediately after said washing; b) than an oxidizing agent in the form of oxygen gas and/or hydrogen peroxide is thereafter admixed to the pulp in an amount of at maximum 2 kg/ton pulp, at which a pressure of 0,1-1,0 MPa and a temperature of 60-90°C are maintained; c) that an acid is added after 15-120 seconds, counted from the charging of oxygen gas and/or hydrogen peroxide for terminating the treatment with oxygen gas and/or hydrogen peroxide and for lowering the pH-value, but without effecting a complete neutralization of the residual alkaline. 2. A method as defined in claim 1, characterized in that the amount of acid per ton pulp added according to c) expressed in hydrogen ions amounts at maximum to H + =• NaOH - 2 k ol H + per ton pulp 40 where NaOH is the charged amount of alkaline expressed in kg NaOH per ton pulp. 3. A method as defined in claim 1 or 2 , characterized in that the acid is added immediately prior to or together with the chlorine dioxide in the second chlorine dioxide step. 4. A method as defined in any of the preceding claims, characterized in that the charge of hydrogen peroxide according to b) amounts at maximum to 1 kg/ton pulp.

Inventors: FAERNSTRAND PER-AAKE, LINDSTROEM LARS-AAKE, NORDEN K SOLVEIG E, SJOEDIN LARS T, FAERNSTRAND, PER-AAKE, LINDSTROEM, LARS-AAKE, NORDEN, K., SOLVEIG, E., SJOEDIN, LARS, T.
Applicants: SUNDS DEFIBRATOR IND AB, SUNDS DEFIBRATOR INDUSTRIES AKTIEBOLAG
Application year: 1991
Main classification: D21C9
IPC codes: D21C9, D21H17