FI60893B - FOERFARANDE FOER BLEKNING AV FIBERMASSA - Google Patents

Description

RSSel Γ · 1 ADVERTISEMENT / nflQi JH * W (11) UTLÄeGNINOSSKIlIFT 6Uöy 0 • C (45) Patent granted on 13 04 1932. <itVg2f Patent iueddelat (51) Ky.lk.Vci.3 D 21 C 9/10 ENGLISH - FIN LAN D (21) i ^ ttitak ^ -N «tweta *. | U5U / 71 * (22) Hakamltptlvl —AMttknlnpd ^ 18.02-7 ^ (23) Alkuptivt — GlWgh * t * «l» f 25 * θ6 · 70 (41) Tullut lulklMluI - Bllvlt oftwitllg 18.02 7 ^ P »t * ntti- j »National Board of Registration (44) Nlhtlvlkslpanon |» KuLJulkalsun pvm.— no fti

Patent · oeh reglttentyrelsen Ansäkan utiagd eeh uti.tkriftan puMicärad 31 · 12 · 01 (32) (33) (31) Pyy4 * tty «tuolsus — Begird prtorltet (71) (72) Torolf Tom Paul LaxSn, Pirkonkuja 5 C, 02700 Kauniainen ,

Suomi-Finland (FI) (7I +) Papula Rein Lahtela Oy (5U) Fiber pulp bleaching method - Förfarande för hlekning av fi henna (62) Share! Sat separated from application 1795/70 (advertisement publication U5TÖ3)

This invention relates to a method for bleaching pulp, in which the reagent solutions used for bleaching are made to flow through the fibrous layer one after the other without intermediate washing, so that every m m of reagent solution displaces the previous solution from the layer.

The object of the invention is to bleach the pulp in such a way that side reactions can be avoided, the consumption of chemicals is low and the bleaching takes place using small amounts of water and a short reaction time.

In the current pulp bleaching, which still has the same characteristics as 80 years ago, the pulp and reagents are mixed, steam is added as needed, and the stock is allowed to react in a cube or tower that actually resembles a continuous coke. The retention time is in most cases hours, when the bleaching agent is almost consumed. In most cases, the reaction is followed by copious dilution with water, transport to a scrubber, and washing. In multi-stage bleaching, this operation is repeated at each stage, thus each needing its own specially designed tower and scrubber.

A predominant feature in the development of pulp bleaching has been that the improvements have been gradual and that the fins have mainly concerned only the details. The major disadvantages of the current bleaching method can be characterized as follows: 1. A very great difficulty for bleachers is the unusually large

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Jutus. Full bleaching of wood pulp requires an average of 149 rrr / t of water. Not even 4-stage sulphite bleaches reach practically less than 90 m / t of pulp. Despite the re-recycling and flocculation of industrial waters, chlorination, pulp transport and washing require large amounts of fresh water. the amount is causing ever-increasing treatment costs due to tightening demands on wastewater.

2. Bleaching is a very time consuming process. This is due to e.g. that the initial concentration of bleaching agent must be so low that only a very small residue remains when the reaction is stopped. Since the wear of the reagent is very rapid at the beginning of the reaction, but slows down sharply as the concentration decreases, this, together with the low initial concentration, greatly slows down the reaction rate. Thus, the bleaching time of fully bleached pulp is 10-18 hours with one step retention time ranging from 1 to 4.3 hours.

3. A very significant proportion of chemicals are consumed in side reactions with reaction products. This takes place partly already inside the fiber, where the lignin components, which have already been activated, e.g. in the time-soluble form, still have to react due to the long retention time. Naturally, the reaction products also react with the chemicals during migration out of the fiber and in the surrounding fluid.

4. Given the way in which the current process has evolved, it is well understood that the technical solutions of bleaching plants leave much to be desired. At present, the technical implementation of bleaching is rigid, i.e. it cannot be said to vary. Each tower is specifically designed for a particular stage of the bleaching combination. Due to the long reaction time, the towers become large and expensive. The reaction times cannot be varied widely and it is not conceivable that any substantial changes could be made to the bleaching combination chosen, as - as stated - the equipment is designed for completely specific conditions. When one third of the otherwise large amount of process water has to be heated to 50-70 ° C, it is understandable that the thermal economy is poor.

60893

As mentioned, there has usually only been an attempt to improve the details of the process. However, a couple of exceptions are known here. One of these is the bleaching method, in which bleaching reagents are added as a gas, and the other is known as dynamic phase bleaching. The latter should be looked at a little more closely. The method is disclosed in British Patent Publication 1,100,174. According to the publication, the biggest retarding factor in bleaching is the immobile liquid film surrounding the fibers. By forcing bleach liquids to flow through the pulp deposit in conventional phase bleaching, attempts have been made to remove this film and it has been shown that significant savings in bleaching time are possible. On the other hand, it is also known that an equally significant saving is possible by mixing the chemicals into the pulp in the usual way, provided that the same level of brightness is satisfied as is achieved by Britt. by the method of I.IOO.174. Attempts have also been made to perform dynamic alternate bleaching very quickly. In that case, however, severe bleaching conditions have had to be used, in which case the viscosity of the pulp has suffered and the brightness has remained low. No attention has been paid to the fact that the bleaching time should be limited so that no adverse side reactions or fiber deterioration occurs.

The invention disclosed herein is intended to obviate the disadvantages of the types of processes discussed above. The invention utilizes the information currently available on the Theology of Pulp Suspensions and the Reaction Kinetics of Bleaching.

The method according to the invention is mainly characterized by what is stated in claim 1.

As bleaching is a heterogeneous reaction, the following steps must be distinguished: 1. Transport of chemicals dissolved in a liquid: a) Transport through a moving liquid layer b) Diffusion through a stationary liquid layer surrounding the fiber c) Diffusion through a solid to the reaction surface 2. Adsorption of chemicals in the solid phase to the surface 3. Chemical reaction with mass components 4. Desorption of reaction products 5. Diffusion of reaction products

The passage of chemicals through the moving liquid layer takes place very quickly and no further treatment is necessary. Diffusion through a stationary liquid film can certainly affect the reaction rate 60893. The entire surface of the fiber is covered by a stationary liquid layer and is particularly thick in the recesses of private fibers such as lumen. Fiber bundles may leave a stationary liquid that prevents reactions long after it has been completed with the individual fibers.

Diffusion through a solid, based on everything, has at least as much effect on the reaction rate as liquid layer law. It is also worth considering the sorption of water molecules into the solid phase. In terms of molecular size, each reagent molecule must diffuse from it for quite long distances. Lignin has been shown to have a relatively high water sorption capacity. This indicates that the interiors of the lignin material are at least partially accessible to the diffusible reagents. On this basis, it cannot be assumed that the bleaching would have a homogeneous reaction surface which, as the reaction progresses, would move steadily towards the center of the fiber.

It is very difficult to understand the role of adsorption in the bleaching reaction. It is known that in heterogeneous reactions, van der Waals adsorption is predominant at low temperatures, while the importance of chemisorption increases with increasing temperature. If the structure of the lignin is porous, it must be taken into account that a diffusion-dependent concentration gradient is formed in the lignil layer in the reaction. It should be assumed that the chemical reaction itself usually occurs so rapidly that sorption or diffusion becomes a retarding factor as soon as a layer of reaction products has formed and the reactive layer moves inwards. Thus, the high initial rate of bleaching reactions during the first minute of the reaction can be explained. The rate of desorption of ions from the fiber cannot be greater than their rate of adsorption to the fiber to maintain concentration equilibrium. If the diffusion at some stage determines the reaction rate - as is likely - the specific rates of adsorption and desorption become equal.

The method now presented is characterized in that two or more types of solution are run in alternating pulses through the fibrous pulp layer or web. The pulses usually have a short duration, suitably e.g. about 1/2 to 3 minutes. and the next pulse always penetrates the previous one in its path. Thus, a high initial reaction rate can be utilized. It can be further accelerated because high concentrations of chemicals can be used momentarily. After all, the pulse nature of bleaching leads to momentary reactions.

5 60893

The invention is illustrated below with reference to the accompanying drawings. They show fig. 1 consumption of reagents according to the invention and according to a conventional method, fig. 2 consumption of the reagent contained in the pulse, fig. 3 ingredients in elution solutions, fig. 4 schematically from the side of the device for carrying out the method * fig. 5 same as above but from above,

Figure 1 shows a typical chemical consumption curve 1 for normal bleaching. In this case, the initial concentration is Cq, and the figure shows that most of the chemical is consumed in a couple of minutes. The figure also shows the consumption curves of pulse bleaching with two types of solution, the wear of which in pulse bleaching is shown by curves 2 and 3. They first show that a higher concentration has been used than in conventional bleaching, e.g. 5 times higher depending on the chemical. in addition, the brackets 2 'and 3' indicate which part of the consumption curve is used for pulse bleaching.

For example, alternating pulses can be used to drive a combination of two pulses, first the oxidant-containing solution through the pulp bed and immediately afterwards the alkali-containing solution which dissolves and transports the reaction products, this combination being run as many times as needed. For example, a combination of three pulses can be used to run the oxidant, alkali, and oxidant or mass stabilizing solution - such as a complexing agent or buffer - repeatedly in succession. Different pulse combinations can be combined to form a bleaching combination.

The aim of the repetitive · pulse combination is to create a kind of total reaction from short, fast and different reactions, in which the sub-reactions alternate rapidly because the general impression is one reaction.

In the reaction layer, the chemical concentration remains high and the concentration of reaction products remains low because the latter is dissolved and transported away at frequent intervals. This affects diffusion and sorption as well as Donnan equilibria in the solid phase.

Furthermore, in pulp bleaching, the bleaching reagent generally reacts significantly more slowly with carbohydrates than with lignin. As a result, the viscosity and strength properties of the pulp are well preserved because only the very fast initial stage of the lignin reaction is used, so that the reaction time is so short that the carbohydrates do not have time to react in a considerable amount.

Our studies have shown that »successive layers of reagent are not said to mix with each other when run on a fiber column. naturally, the boundary becomes somewhat diffuse, due at least in part to intramibular diffusion, so that if the distance traveled by the pulses in the pulp is not too long, they can be kept quite well apart. The different temperatures, specific gravities and viscosities of the solution and / or mixture contained in two successive pulses, as well as the appropriate fiber density in the stock, are important factors here. The fiber consistency may suitably be equal to or greater than 12 yo.

As indicated above, relatively high reagent concentrations are used in pulse bleaching. This means not only an increasing reaction rate, but also that the concentration difference between the fresh and the pulse used, despite the consumption of the reagent, does not play any significant part in the course of the reaction. Because a fairly high pulp consistency is used and no intermediate wash is used during bleaching, it follows that the amounts of liquid are very small compared to the conventional bleaching method.

It was mentioned above as an example that in a pulse combination of two reagents, where the solutions used for each pulse contain their own reagents, the first pulse can activate the reaction surface and the next can dissolve and transport away the reaction products. When the pulses are again separated from the elution stream, e.g. by a three-way tap connected to automatically change position when the redox potential or electrolytic polarization current of the elution solution changes, and a solution from the second pulse in which the solute is more consumed but in which the concentration of solutes is high. Due to its purity, the former solution can be run back to bleaching at any point in the process, but due to its reduced volume and concentration, it is particularly suitable for feeding to bleaching at a later stage. The reagent in the latter solution can be fortified and reused 7 60893 to some extent, after which it can be evaporated, dialyzed or otherwise treated due to its relatively small volume and high solute content.

Fig. 2 schematically shows a pulse entering the mass as an almost rectangular surface 4, a pulse coming from the pulp as a surface 5 surrounded by a solid line, the surrounding dashed lines illustrating how much of the going pulse has been consumed. The wave brackets indicate the pulse section 6, which is reused in the process.

In describing this Fig. 2, as in the present invention, the word pulse is given not only its conventional meaning of pulse and the like, but also the amount of liquid, solution or mixture which, like a pulse, is momentarily forced to react with the fibrous layer. The figure shows how the chemical concentration decreases as the pulse passes through the mass layer nucleus.

Figure 3 shows the concentrations of the solution coming from the pulp, in successive pulses as a function of the flow direction, as follows: the thin line indicates the oxidant concentration 7, the thick line the alkali concentration 8 and the dashed line the organic matter concentration 9.

It can be seen from the presentation that the organic matter containing the reaction products, which is dissolved in the time pulse, has mainly consumed the initial part of the alkali pulse.

The following table compares the parameters of pulse bleaching and conventional bleaching. Only those factors that can be varied during use are included and listed in order of priority:

Pulse bleaching Ordinary bleaching

Chemical concentration, g / l Chemical / dose, # by mass

Temperature, ° C Temperature, ° C

Pulse flow rate in mass in cm / min, represented by vector separation with mass in motion, Reaction time, h according to Figure 4, reference 15

Pulse time, min. Sakeus, i »

Bleaching combination, ie the combined order of reagent and liquids

The possibilities of the method in bleaching are not limited to water And conventional bleaching chemicals. As already stated, process 60893 uses small amounts of liquids and their recovery and fractionation are essentially part of the process. In addition, the reaction zone is isolated from the environment. These factors also allow the use of non-ordinary substances in bleaching; namely, pulses which, if necessary, consist of various organic solvents, such as dioxane, dimethylsulfoxide or oxide, can be included in the valcais combination. the penetration of liquids into the fibers can be easily promoted with surface-active agents and the metallic linains harmful to bleaching can be removed with complexing agents such as EHTA: 11a »l) PTA or NTA. Above and below, the issue is presented with commonly used acronyms, some examples of which are given below.

Common bleaching agents: chlorine, Clg »C

chlorine dioxide, C102 '= D

hypochlorite, C10 ~ = H

alkali, NaOH = E

hydrogen peroxide, H 2 O 2 = P

sodium dithionite Na 2 S 2 O 2 = Dit,

In addition:

EDTA, complexing agent, = S

hydrochloric acid, HG1 = Ac sulfur dioxide SO2 = SO2 water H2O = V /

Pulse or: a repetitive pulse combination is written in parentheses. The index number after the parentheses indicates the number of times the pulse combination is repeated in succession: Full bleach (AcC) j (ED) ^ WS02 lignin-saving bleach: (S JDit) ^ semi-bleach (CAcP) ^ WG02

The device for carrying out the process can easily be designed so that the reaction zone is well isolated from the environment and in a continuous device the mass acts as a shield on both sides of the zone. Thus, the sensitive parts of the device are protected against corrosion and nothing can evaporate from the liquid. Thus, the pulse bleaching method does not require scrubbers or special reactor types, Figures 4 and t>, which. will be explained in more detail later, schematically show as an example the main principle that the required device can be formed from similar interconnected modules. Thus, the plant for this process is flexible so that it can be easily used for different bleaching combinations and modifications, as any of the modules can be easily switched to a different solution.

The present invention provides a method for bleaching in which - water consumption is low - reaction time is short - side reaction between reaction products and bleaching chemicals is reduced - bleaching is flexible * combinations, reagents and liquids can be varied as needed - whole bleaching can take place in the same in a device where the reaction zone is protected from the environment and the most sensitive parts of the device, thus protection against corrosion and not the use of ordinary bleaching solutions is facilitated.

The illustrative examples are intended to illustrate the practical advantages of pulse bleaching. The notations described above have been used in the examples.

Example 1 50 g of unbleached pine sulphate pulp with a kappa of 37 and a viscosity of 1359 ° were chlorinated in the usual manner. Chlorine consumption was 6.9 i »mass. After completion of the reaction, the mass was washed and divided in half.

The other part was further bleached in the usual way with the combination EDED. The mass was then acidified with SO 2 and washed. The pulp was evaluated and brightness and viscosity were determined according to 3CAN grain methods.

The other part was pulsed bleached with the combination (ED) g, The elution solution was continuously determined as ClOg as active chlorine. Post-treatment of the pulp and determination of brightness and viscosity were performed in the same manner as in the previous case.

Pulse whitening Reference protein

(ED) g EDED

The consistency of the stock, # 12 The consistency of the stock, j »10 temperature, ° C 60 temperature, ° C 60

Thickness of the mass track, cm 12

Pulse time, min 1.0 Reaction time / lie, min 60 10 60893

Pulse flow rate cm / min 2.5 0102 concentration in solution g akt Cl / 1 4.1

Total amount of G102 in pulses, Total amount of C102 amount of dosing active Cl

Total ClOg consumption, act. Total C102 “expenditure act. Cl

The amount of Cl as? S. from mass 2.3 amount as # of mass 5.2

NaOH concentration in solution g / l 1.6 total. Amount of NaOH in pulses Total amount of NaOH from dosing mass in 3 * 2 mass, i> from mass 4.0

Brightness, SCAN 85.0 Brightness, SCAN 88.5

Viscosity, SCAN 1290 Viscosity, SCAN 980

Example 2

The devices in which pulse bleaching is performed can be very diverse. The main requirements of such a device are e.g. the following points: -the nozzle structure of the reagent solutions must be such that the solutions do not mix before passing into the mass -the mass must remain in a uniform layer, at a suitable consistency suitably at a consistency of about 15 'fo or move continuously in a continuous device.

One schematic solution of such a continuous device is seen in Figures 4 and 5.

The mass 12 moves in a tube 12 ', in the middle of which there is a longitudinally axial reagent nozzle structure 10, the outer shell of the structure 10 being a wire or the like. Inside the outer shells of the tube 12 'there are compartments 14 with a slit plate 13 on the inner surface.

For example, when the pulp moves from the bottom up and the bleaching assembly is formed of different pairs of pulses, the reagent nozzles are used as follows: The nozzles in the same horizontal plane are divided so that two adjacent nozzles 18, 19 are used for different pulse chemicals. The reagents of the different pulse pairs are arranged on top of each other in different levels. The entire nozzle structure 10 rotates back and forth around the vertical axis at a rate of 90 ° pulses, e.g. once a minute, providing a change in reagent pulses.

Since in the example the mass moves upwards, the reagent solution moves horizontally and when gravity is taken into account, a vector diagram 15 is formed according to Figure 4, in which the vector separation associated with each tray H although a simplification of the drawing. there is only a moth tap drawn in connection with one compartment. with three-way taps, different fluids of different pulses that have passed through the mass can be collected, in their own tanks, and from them for further use or processing. In front of the liquid outlet pipes of each nozzle there is a convex plate 21 which reverses the flow direction of the liquid coming out of the nozzle tube in the direction of the arrows 22, so that the liquid flow after the concave wall 23 in the shape of the paraboloid

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