DE1052797B - Process for bleaching wood pulp - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN M ^ m < PATENT OFFICE

RL. 55C 1

INTERNAT. KL. D 21 d

EXPLAINING EDITORIAL 1052 797

S 42472 IVa / 55 c

REGISTRATION DATE: JANUARY 28, 1955

NOTICE
THE REGISTRATION
AND ISSUE OF THE
EDITORIAL: MARCH 1, 1 9 5 9

It is known to reduce wood pulp with the help of dithionites, such as sodium or zinc dithionite to bleach. This results in an increase in the whiteness that is normally around 60% 6.5 to 7.5%.

For ¹ prevention of corrosion of the metallic used here. Corrosion inhibitors that neutralize the acid have already been added to equipment in the bleach bath. Among the proposed in this context buffers is also calcium phosphate which has the effect that the pulp is colored even not dark, when the _pH value of the pulp to 6.5 or 7 increases.

According to another known process, in the reducing bleaching of ground wood with dithionite, buffers, such as, for example, diammonium phosphate, are added in order to keep the p _u value in the range from 4.5 to 6.

Neither the addition of calcium phosphate nor that of diammonium phosphate to the bleach bath contributed the reducing bleaching with dithionites, however, leads to a greater increase in the whiteness, than that which can be achieved by dithionite alone.

Attempts to increase in Brightness in the dithionite by controlling the p _H -value or the temperature by adding brightening acting agents, etc. have not yet been successful.

It has now been found that the whiteness of the goods can be reduced by bleaching wood pulp with dithionites can increase by. to the Well a polyphosphate not much later than that Dithionite adds · and both additives are kept in a mixture with the product during the bleaching process. Preferably Polyphosphate and dithionite are mixed practically at the same time with the wood pulp.

In order to show that the effect of the process according to the invention cannot be achieved by the known addition of phosphate buffers such as diammonium phosphate, two series of tests were carried out. In the first row, the unbleached wood pulp was treated with 1% sodium dithionite and various amounts of phosphates in distilled water. In view of the use of distilled water, the interfering substances could only come from the wood pulp. The bleaching was in all. Cases at 80 ° C and a Stoffdiohte of 3% in the course of 30 minutes at a p _H of 6 performed. The results can be seen from test series I.

In the second series of experiments, the same method was used, but using factory water, which contained 10 parts ferric chloride per million parts. Test series II shows the results.

Process for bleaching wood pulp

Applicant:

Scott Paper Company, Chester, Del. (V. St. A.)

Representative: Dipl.-Ing. W. Meissner,

Berlin-Grunewald, Herbertstr. 22

and Dipl.-Ing. H. Tischer, Munich 2, patent attorneys

Donald Baxter Sparrow, Moylan, Del.,

Kenneth William Britt, Norwood, Del.,

and James Joseph Eberl, Moylan, Del. (V. St. A.),

have been named as inventors

2 Using
from = 59.8. Using A ^ series of requests I. Sodium- by Natrkim- tripolyphosphate Achieved whiteness tripolyphosphate Added
Phosphate in% 68.4 Using 67.4 of the pulp 69.1 of diammonium 69.5 68.2 phosphate 70.3 0 69.5 67.4 0.1 69.5 66.3 0.2 .— 67.7 0.3 70.1 0.4 0.5 1.0 Whiteness of the Added Achieved whiteness Phosphate in% Using
from Diammonium of the pulp phosphate 0 68.4 0.5 67.8 1.0 68.1 67.8 68.0 68.1 65.9 unbleached pulp = Test series II

50 "whiteness of the unbleached pulp = 59.8.

This means that normal phosphates (diammonium phosphate) increase the whiteness during bleaching do not increase with dithionites, while adding

809 769/315

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set of polyphosphates]! a considerable improvement on normal wood sanding as well as on chemical wood

tion is achieved, especially when sanding with factory water directly from the grinding machine or

works as it does in practical operation. from one remote from the grinding machine

The effect of the polyphosphate appears to come in place of the pulp preparation process on the fact that the resistance of the dithionite in the wood 5 can. The wood pulp coming from the grinder is increased considerably by this addition. sanding can also first be dried and balled It has been found that the dithionite is processed and the pads can be transferred to another transported in the wood pulp in the presence of polyphosphate, where it is crushed and then back to a pulp Is 5 times as stable as without this addition. Sets to be processed. Such a good can also be called Ausman. the polyphosphate is used in the wood pulp as a raw material for the process according to the invention, moderately short time after the dithionite too, so that the only essential thing is that the good during the bleaching dithionite before the addition of polyphosphate, the consistency of the substance is already correct during the process. Generally has the opportunity to decompose, the material density of the invention can be determined proper technical progress no longer necessary. bleaching good in the range of about 0.5 weight goals. 15 percent to the point where an effective

It is known that mixing is no longer possible with unbleached sulfite pulp, e.g. B. 10 or a reddening occurs due to oxidation processes, which is even 15 percent by weight. It is preferable to work on the oxidation of a dye contained in the sulphite pulp at a consistency in the range of about 2 to 5 Ge-Leukof dye. This oxidative weight percentage, as it is the wood pulp coming directly from the grinding reddening according to the information in the literature under 20 machine, and again the catalytic action of heavy metal ions. Goods that have been made into a paste generally already bind these ions by adding a complex-sit. If you use wood pulp of other forming compounds, such as sodium pyrophosphate or parts of the processing process, its sodium hexametaphosphate can be used, so they can no longer exert a lytic effect by simply adding or removing them, so that the reddening of water adjust accordingly,
of the unbleached sulphite pulp by adding the The bleaching temperature is above the room temperature of substances that can be prevented. However, this known temperature, generally above about fact, was able to solve the present 50 ° C, preferably above 57 ° C. The problem cannot be added, since sulfite pulp even from bleaching can still be carried out at 100 ° C; dcr-lignin is freed and therefore has a different combination of high temperatures but no composition than wood pulp and the advantages of this; The preferred temperature range for pyrophosphate or sodium hexameta- is between 71 and 88 ° C. If you work with fresh phosphate to prevent the reddening of un- wood pulp, the slurry, like the bleached sulfite pulp, has no inferences from the grinding machine , in general increased bleaching effect of dithionites on wood because of the heat development during pre-sanding in the presence of polyphosphates taken to the correct temperature,
could be. A water-soluble di-

It is also known to weave straw such as hats, thionite, as it is already used for this purpose.

or loose vegetable fibers for bleaching is known, e.g. B. zinc dithionite, the alkali dithionke, such as

Prepare by first dividing the products in sodium and potassium dithionite 40, ammonium dithionite

one of dithionite and sodium pyrophosphate and the like, sodium and zinc dithionite become special

soaking standing, strongly alkaline bath and then preferred. The dithionite can, the wood pulp in

added by a solid form consisting of dilute sulfuric acid or in aqueous solution

Clarifying bath leads. This pretreatment is followed by. The latter type of addition becomes the

the actual bleaching. Here, the Pyro-45 should be preferred because of better mixing. The amount

phosphate have an influence on the fiber substance. of dithionite depends on the type of pulp,

In this known pretreatment, however, there is the desired increase in the whiteness and

no bleaching instead; for bleaching with other factors known per se. In general

Thionites can not be known to be strongly alkaline, it is at least about 0.25 percent by weight of the

Solution to be run. Also that will be for 50 dry pulp. With 5% di-

the pretreatment used dithionite and the thionite or beyond work; however achieved

Pyrophosphate by the subsequent treatment in one with amounts above about 3% no significant

the sulfuric acid clarification bath before the actual borrowed advantage more. Preferably the amount is

Bleaching again from the bleached material from dithionite about 0.5 to 1.5%, based on the

washed. 55 dry weight of pulp. ·

There are two types of wood pulp: firstly, the polymal wood pulp to be added within the scope of the invention, secondly, chemical wood pulp. Phosphate is a water-soluble polyphosphate the all-normal wood pulp is produced by simply grinding the common formula (M ₂ O) _x P ₂ O ₅ , in which M is a wood, where the wood shredded into fibers means valuable cation, which is the compound. Chemical pulp is made by giving 60 water solubility, and χ is less than 3. When the wood is first chopped into chips, these play for such monovalent cations are water short with chemicals such as sodium sulfite and sodium, ammonium, the alkali metals such as sodium and trium bicarbonate, at higher temperatures and potassium and the like. can also work with mixtures of pressures, whereby the cutlets are somewhat softened polyphosphates. The polyphosphates can be used, and the chips are then grinded into fibers. 65 be crystalline or glass-like. Examples of both types of wood pulp have high lignin polyphosphates that can be used in accordance with the invention are gnawing contents and therefore, compared to normal trium pyrophosphate Na ₄ P ₂ O ₇ (x = 2), sodium tetra-cellulose, difficulties in bleaching, phosphate Na ₆ P ₄ O ₁₃ {x = 1.5), sodium tripolyphosphate ■ The invention is applicable to wood pulp, which can come from phat Na ₅ P ₃ O ₁₀ O = 1.67), sodium hexapolyphosphate from various sources, namely both 70 Na ₈ P ₆ O ₁₉ {x = 1.33), sodium heptapolyphosphate

z / y /

Na ₉ P ₇ O ₂₂ (χ = 1.29) and sodium decapolyphosphate Na ₁₂ P ₁₀ O ₃₁ (# = 1.2), where the sodium ion is mentioned for M only as an example. Of the polyphosphates mentioned, those in which the value is between 1 and 2 and M is an alkali metal, in particular sodium, are preferred. Particularly preferred are sodium tetraphosphate and sodium pyrophosphate, especially the former.

Just like dithionite, polyphosphate can also the wood pulp in solid form or - preferably - added in aqueous solution. Here, too, the amount depends on the type of wood sanding and the desired whiteness. Most of time you work with at least 0.1 percent by weight, based on the dry wood pulp. Amounts of 5% and more are allowed; however, amounts in excess of 2 to 3% do not provide any significant benefits. A'Or preferably work with about 0.3 to 1% polyphosphate, based on the dry weight of the wood pulp.

The polyphosphate can be added to the wood pulp at any time before or together with the dithionite will. However, if it is added later, the addition must immediately follow that of the dithionite follow before a substantial amount of the dithionite decomposes. The two additives are preferably mixed at the same time as wood sanding, d. i.e., you can use dithionite and polyphosphate separately, however practically at the same time add the wood pulp, or you can first use both additives, z. B. in water, Mix together and add the mixture to the wood pulp. The latter method is preferred.

The intimate mixing of dithionite, polyphosphate and wood pulp required for bleaching can be carried out on different ways, e.g. B. with the help of paddle mixers. After mixing, the remain Fabrics in contact with each other until the desired bleaching effect is achieved, something of. the Amount of dithionite and polyphosphate, the bleaching temperature, the type of wood pulp, the desired Whiteness and other factors known to those skilled in the art depends. In general, the bleaching time lies in the range of a few, e.g. B. 3 to 90 minutes or longer. Among the preferred ones given above Working conditions a satisfactory bleaching is obtained in 10 to 30 minutes.

The mixture should not be strongly alkaline during the bleaching process, that is, the _pH-value should not lie substantially above 8.5. Is preferably carried out at an acidic p _H. At a _pH value of 3 to obtain a satisfactory bleaching. The _pH value in each set depends also on the type of dithionite and polyphosphates used. Do you work z. B. with an alkali oil phosphate, the pjj value can be slightly higher than when using one of the other specified cations. In most cases the _pH value is between about 4 and 6.5, with a p _H is preferably 5 to 6

An advantageous method of mixing dithionite and polyphosphate with the wood pulp and. to the Carrying out the bleaching consists in that the combined bleaching agent in aqueous solution continuously is introduced into a continuous stream of pulp. The mixture is then in kept vigorous agitation, the bleach mixture being intimately mixed with the wood pulp, and at the same time in the form of a continuous flow, preferably in a streamlined, laminar flow propelled along a limited, airtight conduit, the flowing mixture being practical fills the entire cross-section of the flow line. The mixture is continuously closed Cover the flow path until the desired bleaching effect is achieved. Preferably done the flow through the airtight duct on a vertical flow path on which the Mixture alternately increases and decreases, z. B. in a series of vertical cascaded Towers.

ίο After bleaching, the wood pulp suspension can on to. known type dewatered or processed directly into paper. For example you can Dilute the pulp suspension, pass it through a splinter catcher, where none of it has already been done removed pieces of wood are removed, and then further, e.g. B. to a consistency of 0.3 to 0.4%, dilute. The diluted wood pulp is then optionally passed through a fine sieve and drains him z. B. up to a consistency of about 25%, whereupon it is down to a consistency of about 50% presses. In addition to dithionite, other bleaching agents can also be used within the scope of the invention and use other auxiliaries in addition to the polyphosphate.

' Example

Wood pulp is at a temperature of about 80 ° C and a consistency of about 3% with a aqueous solution of. Sodium dithionite and sodium tetraphosphate are added. The amount of dithionite is 1%, that of tetraphosphate 0.5% of the dry weight of the wood pulp. The fabrics will vigorously mixed, after which the cut is left to rest for about 5 minutes. Then one dilutes the ground to a density of 0.3% and processed it in a known way to paper. The paper has a whiteness of 69%, while the unbleached wood pulp only has a whiteness of 58%.

If the same wood pulp is bleached without the addition of polyphosphate, the whiteness of the paper made from it only 4 units more than that of unbleached wood pulp.

In another case, the whiteness of a wood pulp could be reduced by the bleaching according to the invention increase with sodium dithionite and sodium tetraphosphate by the above procedure from 61 to 72%, while bleaching with dithionite alone resulted in an increase in whiteness of only 7.7 units.

Claims (4)

Patent claims: 1. Process for bleaching ground wood with a water-soluble dithionite with the addition of a Phosphates, characterized in that a polyphosphate is not added significantly later to the bleached material when the dithionite is added and both additives during the lead milling process with the good in Mixture are kept. 2. The method according to claim 1, characterized by a temperature of the mixture of substances between 71 and 88 ° C and the use of a sodium polyphosphate with a molar ratio of , O ₅ between about 1: 1 and about 2: 1. 3. The method according to claim 1 and 2, characterized in that the water-soluble dithionite Sodium dithionite and, as sodium polyphosphate, sodium tetraphosphate is used. Na ₂ O 7 8 4. The method according to claim 1 to 3, characterized in that the documents considered: indicates that the amount of dithionite is British Patent No. 13,753 of 1913; between about 0.25% and about 5%, preferably 1913; between 0.5 and 1.5%, and that of Na USA Patents Nos. 2 071304, 2 071307, trium polyphosphate between about 0.1 and about 5,2 290,601, 2,450,034; 5%, preferably between 0.3 and 1%, based on weekly paper for paper production, 78th year, on the dry weight of the wood pulp, measured 1950 p. 546; 79th year, 1951, p. 158; 81st year, will. 1953, p. 570. © 809 769/315 3. 59