FI93749C - Process for bleaching mechanical pulp - Google Patents

Abstract

Bleaching of pulps of mechanical origin in which the pulp is treated with a metal-complexing or sequestering agent and is then washed with an efficiency higher than 95 % before being subjected to the action of hydrogen peroxide at a temperature above 100 DEG C and not exceeding 250 DEG C at the pressure of steam at the said temperature for 1 to 30 minutes, not conjointly with a mechanical beating action.

Description

93749

Method for bleaching mechanical pulp

The invention relates to a process for bleaching a mechanical pulp, in which the pulp is treated with hydrogen peroxide in the presence of an alkaline substance and saturated water vapor. Mechanical pulp means mechanical, thermomechanical, chemimechanical and chemithermomechanical pulps used in papermaking and similar industries. Hereinafter, all these masses are referred to as "masses of mechanical origin".

Industrially, said pulps are most often prepared using wood, usually in the form of chips, by mechanical defibering of the lignocellulosic pulp, for example in a grinder or double disc mill, and the treatment may involve, depending on the desired pulp, pretreatment with steam and / or a chemical reagent such as sodium sulphate.

The dry matter yield in such a process is always high and can be 90%, even more than 90%.

Pulps of mechanical origin, which may also be referred to as "high yield pulps", must be effectively bleached with minimal material loss in order for the resulting products to meet the quality and economy requirements of the industry.

Pulps of mechanical origin have been known to use an oxidizing agent such as hydrogen peroxide in an alkaline environment.

It is well known that the optimum implementation temperature for such conventional hydrogen peroxide bleaching is about 60-70 ° C, see for example C. W. Dence and S. Omori,

Tappi Journal, Oct. 1986, 120-125 and FR Patent Publication 2371544. At temperatures of 100 ° C or above, effective bleaching is impossible.

Above 100 ° C, the effect of bleaching can even disappear due to the simultaneous rapid decomposition of hydrogen peroxide and the particularly harmful effect of water vapor, as shown, for example, in the FR patent and article already mentioned: W. E. Lunan, K. B. Miles and W. D. May,

Proceedings Tappi, 1983 Pulping Conference, (Book 1), 239-253.

Furthermore, according to the aforementioned FR patent, valid bleaching cannot be carried out at a temperature of 100 to 150 ° C, in a water vapor atmosphere, using hydrogen peroxide, in the presence of an alkaline substance, unless the effect of hydrogen peroxide on the bleaching agent is for a fraction of the time.

The process according to the invention provides effective bleaching, although it has steps which are known to have prevented a good result.

The invention relates to a process for bleaching pulps of mechanical origin, in which hydrogen peroxide acts in the presence of an alkaline substance and saturated water vapor. The process is characterized in that the pulp is treated with a metal complexing or sequestering agent at a pH of 9 or less, then washed with a washing efficiency of more than 95% and finally subjected to hydrogen peroxide at 145-245 ° C under saturated water vapor pressure. at that temperature for 1 to 30 minutes, with the hydrogen peroxide treatment taking place at a different time from the mechanical defibering.

In treatment with a complexing or sequestering agent, this agent is used in amounts corresponding to 0.1 to 1% by weight of the dry mass. Unless otherwise stated below, all amounts of substances are likewise expressed as a percentage by weight of the dry mass.

The complexing or sequestering agent is most often selected from the group consisting of sodium tripolyphosphate, sodium tetrapyrophosphate, and sodium salt forms of citric, nitrilotriacetic, ethylenediaminetetraacetic, or diethylenetriamine pentacetic acids.

The treatment with a complexing or sequestering agent is generally carried out at a pH in the range 4-8, i 4 93749 suitably in the range 6-8. However, the result of the method according to the invention does not depend on this pH.

The process is preferably carried out at a temperature below 100 ° C, but generally above 20 ° C, for example 50-95 ° C, in order to maintain the rate of complexation or sequestration without the use of pressure.

The consistency, the weight percentage of dry mass in the treatment medium, when varying from treatment with a complexing or sequestering agent, can vary considerably, from about 5% to about 30%. In most cases, it is in the range of 10 to 15, so that the economy and efficiency of the next wash later are good.

The average treatment time with a complexing or sequestering agent depends on other factors. It usually ranges from about 5 minutes to 2 hours.

• a ·: · Washing refers to the operation of removing more or less completely, and here effectively, the water present in the pulp. The removal can be carried out, for example, by pressing the pulp against the filters or by possibly repeated dilution and compression of the pulp, in which case water is most often used as diluent.

After treatment with a complexing or sequestering agent, the washing is generally carried out at 20-37 ° C at 5-90 ° C, often mainly at 20-60 ° C for economic reasons.

Efficiency means, expressed as a percentage, the degree of removal of the liquid phase present in the pulp before washing.

When the pulp is pressed, the change in consistency from 10% to 20% corresponds to a washing efficiency of 55.5%, the transition to a concentration of 10% by dilution and pressing to 20% corresponds to a washing efficiency of 80% of the original pulp.

Furthermore, for economic reasons, but also in order to more reliably guarantee the bleaching efficiency, the temperature at which the hydrogen peroxide acts in the process according to the invention should preferably be above 120 ° C, for example 140 ° C or more.

The amount of hydrogen peroxide used is the same as the amounts accepted for conventional bleaching of pulps of mechanical origin, i.e. about 0.5 to 10%, most often more than about 2%.

A suitable early agent present with hydrogen peroxide for carrying out the invention is sodium hydroxide, the amount of use is generally about 0.25 to 5%, the weight ratio of sodium hydroxide to hydrogen peroxide is preferably 0.2 to 1.

In the process according to the invention, the duration of action of the hydrogen peroxide is, on the one hand, long enough for the effect of the substance to be as effective as possible and, on the other hand, preferably shorter than in conventional bleaching. Usually it doesn’t take more than 15 minutes and often even just 5 minutes.

As already stated, the effect of hydrogen peroxide takes place in an environment where the pressure is practically the same as the pressure of saturated water vapor at the selected temperature.

The pressure is usually 2-40 absolute bars.

The consistency of the pulp in the presence of hydrogen peroxide is preferably chosen to be as high as possible when carrying out a mixture in which the pulp is in a satisfactorily homogeneous form with the other substances present. Due to this property of the mixture, the consistency is generally limited to less than 30 3 », even only 25%. However, it is rarely less than about 10%.

The present invention provides better results in bleaching pulps of mechanical origin without the presence of hydrogen peroxide stabilizing compounds being necessary for the effectiveness of hydrogen peroxide. However, the efficacy may be improved in the presence of compounds such as sodium silicate, most often 2-6%, magnesium sulphate-like magnesium salts, usually 0.05-0.5%, or poly (<* - hydroxyacrylic> acid derivatives, usually 0.05 - 2 %, for example Clarene R.

The process according to the invention can be carried out continuously 93749 7 or non-continuously in any apparatus with a stirrer, usually of the kneader type, in which treatment with a complexing or sequestering agent and mixing of the starting mass or washing with hydrogen peroxide and alkaline, washing device, e.g. filters, presses and also , for the hydrogen peroxide action phase, a device that is pressure resistant and water vapor! ivis, for example a tubular reactor with a screw-type pulp transport system and an autoclave.

In the light of the following illustrative non-limiting examples, the invention may be considered. Some examples are comparative examples.

In the examples: - the amounts of substances involved in the reaction are given as previously expressed as a percentage by weight relative to the dry mass, *.

- diethyltriamine pentaacetic acid (DTPA) is used in the form of sodium salt in aqueous solution at 40% by weight, amount means the amount of solution, - silicate means aqueous sodium silicate, density 1.33, amount means this solution, - the abbreviation LS stands for Clarene R, 35% by weight of peroxide, expressed as pure hydrogen peroxide, as well as the hydrogen peroxide used, - the pulp to be bleached may be treated with a complexing agent and washed before being exposed to hydrogen peroxide. In the negative case, the example is given for comparison and is denoted by c. If the treatment is carried out, i.e. the process according to the invention is followed, the treatment of the pulp is carried out in a mixer, 0.5% DTPA, duration 15 minutes, temperature 60 ° C in Examples 1-11, 90 ° C in Examples 12-14, pH in all cases 4 -8, the same as in one example given and in the example given for comparison, consistency 10%. Subsequent washing, always with an efficiency of more than 95%, is carried out at 60 ° C, dilution with water and filter pressing of the DTPA-treated pulp, - the starting pulp or the pulp washed after DTPA treatment is mixed with hydrogen peroxide, sodium hydroxide and water to the desired consistency, here %, if necessary with silicate or Clarene R, is stirred for at least 5 minutes at 20 to 25 ° C after the pulp has first been exposed to hydrogen peroxide in a pressure-resistant autoclave-type reactor with pulp feed and resistant to pressurized water vapor at the selected temperature T , 11 93749 9 - the degree of whiteness of the mass obtained after depressurization and rapid cooling of the autoclave is measured (457 nm) as a percentage using an Elrepho-type Carl Zeiss spectrophotometer.

Examples 1-11

In each example, 100 g of mechanically ground resinous wood, a whiteness of 53.7% and a dry weight of 38.2 g are bleached.

In each example, the conditions under which hydrogen peroxide acts in the process according to the invention or in the process given for comparison, as well as the results obtained, are shown in Table I on the following page: ♦ * * 93749 10 \

TABLE I

Experiment T Duration ΗζΟζ NaOH Si li- LS Whiteness- Used No. ° C min. degree of H2O2 1 145 5 5 2 78.5 2.55 2 145 5 5 2 4 80 2.3 3 145 5 5 4 0.5 81 3.2 4 145 15 5 2 77 3 5 180 2 3 2 75, 5 1.8 5c 180 2 3 2 70 2.45 6 180 2 5 2 77 2.4 7 180 2 10 2 78.5 4 7c 180 2 10 2 75.5 4 8 180 2 10 4 79 8 9 180 2 10 4 0.5 82.5 4.7 10 245 1 5 2 77.5 3.05 11 245 1 54 0.5 79 3.25

From the above examples, it can be seen that the use of the method according to the invention gives a clearly improved degree of whiteness, which can be about 80% and even considerably exceeds it under favorable conditions for implementation.

93749 11 With this improvement, the consumption of hydrogen peroxide will also be significantly reduced or at least not increased.

The degree of whiteness has been achieved with considerable savings in hydrogen peroxide.

In the examples according to the invention, there is no significant variation in the dry matter weight of the pulp.

Examples 12-14

In each example, 100 g of a chemithermomechanical sulfite pulp, a whiteness of 59.6% and a dry weight of 40 g are bleached.

In each example, the conditions under which hydrogen peroxide acts in the process according to the invention or in the process given for comparison, as well as the results obtained, are shown in Table II on the following page: ί 12 93749

TABLE II

Experiment T Duration HzOz NaOH LS Whiteness- Used no ° C min. degree HzOz 12 150 2 5 4 0,5 78 3 13 180 2 5 4 0,5 79 3,3 13c 180 2 5 4 0,5 74,5 4

The observations made on the basis of Experiments 1-11 are still valid.

Claims (10)

A process for bleaching mechanical pulp, wherein the pulp is treated with hydrogen peroxide in the presence of an alkaline substance and saturated water vapor, characterized in that the pulp is treated with a seed piexing or sequestering agent for metals at a pH less than or equal to 9, then washed with a washing efficiency exceeding 95% and finally subjected to the effect of hydrogen peroxide at a temperature of between 145 ° C and 245 ° C at the pressure of saturated water vapor at said temperature for a period of time of between 1 and 30 minutes, the treatment with hydrogen peroxide does not coincide with a mechanical fiber. Process according to claim 1, characterized in that the amount of the complexing or sequestering agent is between 0.1% and 1%. Method according to claim 1 or 2, characterized in that the treatment with the complexing or sequestering agent is carried out at a pH of between 6 and 8. Method according to any of claims 1-3, characterized in that the treatment with the complexing 93749 or sequestering agent is carried out at a temperature between 50 ° C and 95 ° C. Process according to any of claims 1-4, characterized in that the washing is carried out at a temperature of between 20 ° C and 90 ° C. Process according to any of claims 1-5, characterized in that the amount of hydrogen peroxide is between 2% and 10%. Process according to any one of claims 1-6, characterized in that the alkaline substance present when the hydrogen peroxide is allowed to operate, sodium hydroxide is used in an amount of from 0.25% to 5% and with a weight ratio to the hydrogen peroxide of between 0.2 and 1. Process according to any of claims 1-7, characterized in that the time period during which the hydrogen peroxide is allowed to operate is less than 15 minutes. 9. A process according to any one of claims 1-8, characterized in that the hydrogen peroxide may act in the presence of a compound having a stabilizing effect thereon. Process according to claim 9, characterized in that the compound having a stabilizing effect is selected from the group consisting of sodium silicate, magnesium salts and poly (a-hydroxyacrylic) acid derivatives.