FR2497526A1 - Blanching bagasse pulp using oxygen alkali mixt. - by introducing oxygen at hot alkali mixt. under pressure then blanching including chlorination steps (SE 24.5.82) - Google Patents

Abstract

Method of blanching bagasse pulp using an oxygen-alkali mixt. comprising introducing oxygen into an alkaline medium with pressure and heat, then effecting the blanching in a number of steps, including chlorination and/or treatment with ClO2 or H2O2; alkaline extn.; and further treatment with hypochlorite, and/or ClO2 and/or H2O2. Paper pulp can be made from sugar cane bagasse with a lowering of atmospheric pollution by chlorine and its derivs. since bleaching if effected with oxygen. The process reduces the consumption of chemicals appreciably, together with the minor costs of conventional bleaching.

Description

Bagasse pulp bleaching process
using an oxygen-alkali mixture.

 The invention relates to the production of bagasse chemical pulp bleached by treatment with an oxygen-alkali mixture (O / A) and then bleaching in several stages. It also relates to the application of such pulps in the pulp and paper industry, in particular for obtaining many types of paper. The process for obtaining bleached pulps of the invention provides economic advantages and the reduction of atmospheric pollution compared to other methods of bleaching the pulp.

Obtaining wood pulp sulfate bleached using an oxygen-alkali mixture is described in particular by N. Hartler; H. Norrstrom and S. Rydin,
Svensk Papperstidning, arg. 73, Contribution to the problems of delignification of the sulphate pulp in wood with oxygen-alkali (N. Gajdos; Papir a
Celulosa c. 3 1973), Improvement in oxygen bleaching (I. Croon and DH Andrews Tappi v. 54 NO 11 1971).

 The techniques closest to this process are as follows: 1. Improvement in oxygen bleaching.

 This work carried out by I. Croon and DH Andrews (Tappi V. 54 N "11 1971) consists in demonstrating the possibilities of application of oxygen bleaching to wooden chemical pulps followed by multi-stage bleaching using different sequences such as OC / D (oxygen, chlorination with addition of chlorine dioxide); EDEC (alkaline extraction, dioxide, alkaline extraction, dioxide); ODED (oxygen, dioxide, alkaline extraction, dioxide). process variables is similar to that used in the development of the work of the applicant except for the consistency, in the case of the invention which is lower and the percentage of magnesium carbonate which is greater.

 It has been verified that it is possible to obtain chemical pulps of high quality wood by appropriately controlling the variables of the treatment step with oxygen. In addition, there is a significant reduction in air pollution and lower production costs.

2. Whitening with oxygen.

The modification of the bleaching technology is planned in the Cellulose Plant of Aquitaine (Pulp and Paper International March 1975) thanks to the addition of a bleaching stage O / A with which the following advantages are obtained - reduction the color of the bleaching effluents
70 X; - reduction of BOD5 by 50% (biological demand for oxy
gene) - improved gloss stability - reduced number of color reversals - possibility of whitening pulps with greater
initial Kappa number; - reduction in the consumption of chemicals
45% mainly chlorine.

 According to the most remarkable aspect of this process, one can use the hard pulp of wood whose morphological characteristics are similar to those of bagasse.

 But the process of oxygen bleaching of chemical wood pulps requires draconian conditions for the bleaching of bagasse pulps: it indeed requires the use of numerous bleaching sequences, more than 4 steps, which increases the cost of money laundering and air pollution.

 In the case of bagasse, it has been verified that 4 steps correspond to what is necessary to obtain a commercial level of whiteness.

 The object of the invention is to provide a suitable technology for the implementation of a process comprising treatment with an oxygen-alkali mixture, applied to a new raw material (the bagasse of sugar cane).

 With the use of this process, it is possible to considerably reduce the atmospheric pollution which is basically caused by chlorine and its derivatives, which is most important if one takes into account the limited river capacities, which makes it necessary to recover the waters for reasons of economy.

 The invention provides other advantages which can be attributed to bleaching with the aid of oxygen, appreciable reduction in the consumption of chemicals (essentially chlorine) and lower production costs with regard to bleaching. conventional.

 It should be noted that the similar solutions proposed for wooden pulps are not possible for bagasse, since they cause serious problems concerning the strength properties.

 In accordance with the invention, in the above process, sugar cane bagasse is used as a raw material.

 It is important to note that similar solutions provided by wooden pulps do not apply in the case of bagasse, since they cause serious problems concerning the resistance properties of the pulps.

 According to the process of the invention, the bagasse pulps are subjected to the action of oxygen in an alkaline medium with heating and pressure, then to a bleaching treatment in several stages comprising a first stage of chlorination and / or treatment with chlorine dioxide or hydro-gen peroxide, a second stage of alkaline extraction and, thereafter, stages of treatment with hypochlorite and / or chlorine dioxide and / or hydrogen peroxide.

 In one embodiment, use is made of pressures of the order of approximately 4 to 10 × 10 Pascal at temperatures of the order of 105 to 1250 ° C. and a step of treatment with oxygen. The duration of the reaction is advantageously from 30 to 90 minutes approximately for this last step.

 Alternatively, an acid pretreatment is followed followed by a treatment step with oxygen using sodium hydroxide at a concentration of 2 to 6.8% on dry fibers, or other alkali metal hydroxides, to obtain a basic medium. As degradation inhibitors, it is advantageous to use magnesium salts and this at a rate of 0.5 to 3% relative to the dry pulp.

 The process of the invention is advantageously carried out using 1 to 3%, relative to the dry pulp, of chlorine in the chlorination step.

The duration of this chlorination step is of the order of 30 to 60 minutes.

 The percentage of NaOH in the alkaline extraction stage is advantageously from 0.5 to 3 × approximately relative to the dry pulp and the retention time between 30 and 90 minutes approximately.

 In the hydrochlorination stage, it is appropriate to use chlorine at a rate of approximately 0.4 to 1.6% relative to the dry pulp and to operate at a pH between approximately 10 and 12 with a duration of retention of approximately 90 to 180 minutes.

 Preferably, in the first step, chlorine is used in an amount of 0.2 to 1 X and, in the other steps, in an amount of 0.2 to 0.8-X.

 Hydrogen peroxide is advantageously used in the process of the invention at a rate of 0.2 to 1.2 X. The operation is carried out at a pH of 9 to 11 during the step of treatment with the peroxide. 'hydrogen.

 The characteristics of bagasse pulp bleached with this process are as follows: shine 85-90 Elrepho units, viscosity 22-28 cp, a tear factor greater than 55, finding a low BOD and QOD value in the residual liquors.

EXAMPLE.

A bleached pulp is subjected to an acid pre-treatment before the start of bleaching. The conditions used for treatment are as follows
Sulfuric acid (H2SO4): 0.5-2% (BS)
Ambient temperature
Duration: 5-30 minutes
Consistency: 3-10%
At the end of this treatment, the pulp is washed, centrifuged and homogenized to start the bleaching process.

 Treatment stage with an oxygen-alkali mixture.

A liquor composed of treated water, an alkali hydroxide and magnesium salts is prepared, according to the quantities below
Hydroxide: 1.2-6.8 X
Magnesium salt: 0.5-3%
Treated water: What is needed to get
a consistency of 10-20% in the
treatment.

 The liquor and the pulp are preheated to 80-900C and mixed homogeneously by transferring them to a device allowing the digestion of the mixture, the temperature of which is around 90-1200C.

Oxygen is then breathed in to reach a pres
5 2 sion from 4 to 10 x 10 Pascal (4-10 kg / cm) and the temperature rises to 105-1250C (the rise time did not exceed 5 minutes in any case). The suspension is therefore retained in the digester for 30-90 minutes and at the end of this time, it is blown, then the pulp is washed and diluted to continue the process.

Chlorination step.

 The chlorination step is carried out by rapidly and uniformly mixing the pulp and the chlorine (1-3 on dry pulp). The suspension obtained is then retained for 30-60 minutes and finally the pulp is washed and thickened for the next step.

 alkaline extraction step.

The liquor of this stage was prepared by mixing treated water and alkali hydroxide in the following proportions
Hydroxide: 1.2-6.8
Treated water: sufficient quantity for a consistency
10%.

 The pulp and the liquor previously heated to 50-700C are mixed and the suspension obtained is retained for 0.5-1.5 hours. The pulp is then properly washed to start the hypochlorination step or treatment with chlorine dioxide.

Treatment step with chlorine dioxide.

 The pulp from the alkaline extraction of hypochlorination or from an earlier stage of treatment with an anhydride is preheated to 70-800C.

The liquor composed of treated water, hydroxide and chlorine dioxide is then mixed in the following proportions
Chlorine dioxide: 0.2-0.8 (dry basis or bs)
Hydroxide: 0.1-0.4% (bs)
Treated water: sufficient quantity for a consistency
12 X
Retention time: 2-3 hours
The bleached pulp is washed and subjected to a treatment with S02 if it is the final stage in the following proportions
Sulfur dioxide: 1-2%
Consistency: 5-10 minutes
HyDo-chlorination stages.

 The pulp from the alkaline extraction of a treatment step with chlorine dioxide or from prior hypochlorination, is preheated to 35-4o0C and mixed with the liquor at the same temperature.

The bleaching liquor was prepared with treated water and sodium or calcium hypochlorite solution in amounts to fix the following conditions
Hypochlorite: 0.4-1.6% (bs)
Consistency: 12 X
The pH is suitably fixed at values of the order of 10 to 12.

 A time (system) is retained between 90 and 180 minutes and at the end the pulp is washed and treated with 502. If it is the final step, we operate according to the conditions indicated in the treatment step with chlorine dioxide.

 Treatment retreat with a peroxide.

 The liquor of this stage is suitably prepared as follows: treated water, sodium hydroxide or sulfuric acid, depending on the peroxide used, sodium silicate, magnesium salts and peroxide.

Peroxide: 0.2-1.2%
Sodium silicate: O, 1-1%
Magnesium salts: 0.04-0.08% DH: 9.0-11
Consistency: 10-18 X
Temperature: 60-7O0C
Duration: 50-180 minutes
The results given in Table 1 make it possible to make a comparison between the results with this oxygen-alkali bleaching process followed by other stages and a conventional bleaching. As can be observed, the quality of the pulp obtained with the O / ACED sequence (oxygen / alkali, chlorine, alkaline extraction, dioxide) can compete with that of bleached pulp with a conventional CEHD sequence (chlorine, alkaline extraction, hypochlorite, dioxide), while obtaining an appreciable saving of chemicals in the first case.

1. With the use of the bleaching step using an oxygen-alkali (O / A) mixture, the invention provides an acceptable solution to one of the greatest problems of current bleaching factories pulp, that of ambient pollution, because it remarkably reduces the biological demand for oxygen in the process.

This aspect cannot be assessed with absolute figures in order to avoid too detailed information, but it is closely linked to the reduction in the number
Kappa during this stage (between 40 and 60%). This fact is reflected in practice by the simplification or elimination of the effluent treatment plant.

2. There is also a decrease in the charge of chemical reagents in the steps subsequent to bleaching. Oxygen reduces 50% of the amount of chlorine needed to complete the bleaching in the following steps.

3. It reduces the ambient pollution in the preliminary stage of pulp treatment by using a pulp with a higher Kappa number than that of conventional chemical pulps.

COMPARISON BETWEEN WHITENED PULPES OF THE BAGASSE
AND WOOD
Parameters Softwood Hardwood Bagasse
NaOH (%) 2 - 5 - 1.2 - 6.8
Consistency (X) 25 .- 30 21 - 22 10 - 15
Temperature (OC) 90 - 120 120 105 - 125
Near. Oxyg. (Pascal) 2 to 8 x 105 8 to 10x105 4 to 10x105
Duration (min.) 60 30 - 50 30 - 90 MgCO3 (t) 0.05 0.05 - 0.2 0.5 - 3
Final kappa 17 10 10 - 12
Shine (Elrepho) 54.5 - 52 - 58
Viscosity (g / cm2) 955 - 761 - 816
F. Tear 160 - 55 - 60
L. Break (m) 7100 - 2000 - 3000
G. Crushing (OSR) 20 - 18 - 21

Claims (8)

 1. Process for bleaching bagasse pulps using an oxygen-alkali mixture, characterized by the use of oxygen in an alkaline medium with heating and pressure, followed by bleaching in several stages, comprising a first stage of chlorination and / or treatment with chlorine dioxide or hydrogen peroxide, a second stage of alkaline extraction and then stages of treatment with hypochlorite and / or chlorine dioxide and / or hydrogen peroxide.  2. Method according to claim 1, characterized by the use of pressures between 4 and 10 x 105 Pascal, about temperatures of the order of 105 to 1250C, a treatment step with oxygen and a reaction time of 30 to 90 minutes for this step.  3. Method according to claim 1 or 2, characterized by an acid pretreatment followed by a step of treatment with oxygen using sodium hydroxide at a concentration of 2-6.8% on dry fiber or other hydroxides of alkali metals to obtain a basic medium, as well as by the use of magnesium salts, as degradation inhibitors, at a rate of 0.5-3 relative to the dry pulp.  4. Method according to any one of claims 1 to 3, characterized by the use of a percentage of chlorine in the chlorination step of the order of 1 to 3% relative to the dry pulp and a duration of chlorination on the order of 30 to 60 minutes.  5. Method according to any one of claims 1 to 4, characterized by the use of a percentage of NaOH in the alkaline extraction of approximately 0.5-3 relative to the dry pulp and a retention time between 30 and about 90 minutes.  6. Method according to any one of claims 1 to 5, characterized by the use of a percentage of chlorine in the hypo-chlorination step of about 0.4-1.6 Yo compared to the dry pulp , a pH between 10 and 12 and a retention time between 90 and 180 minutes approximately.  7. Method according to any one of claims 1 to 6, characterized by the use of chlorine at a rate of 0.2-1 Yo in the first step and 0.20.8 Yo in the other steps.  8. Method according to any one of claims 1 to 7, characterized by the use of hydrogen peroxide in an amount of 0.2-1.2 X and a pH between 9 and 11 for this step.