DE1076485B - Process for the removal of silica from alkaline cellulose waste liquors rich in silicate, especially in continuous operation - Google Patents

Description

GERMAN

The invention relates to a process for the removal of silica from high-silicate, alkaline acids Pulp waste liquor using soluble magnesium compounds.

The shortage of high-quality fiber raw materials is forcing all pulp producing countries without exception for the increased utilization of annual plants such as wheat, rye, rice and corn stalks as well Bamboo, bagasse and reeds.

These raw materials are mainly digested using the alkaline sulphate process, which means that the SiO ₂ content in these plants is dissolved and is found in the black liquor. In the further course of the waste liquor work-up, the silica content leads to considerable difficulties. During evaporation, encrustations occur on the heating surfaces of the evaporator and the resulting thermal losses and undesirable downtimes caused by cleaning periods. Furthermore, when the waste liquors are burned, the high silicate content of the melts can considerably increase their flow point and sometimes lead to an additional need for additional firing. Great difficulties arise especially during the causticizing; _{If the SiO 2} content is too high, the sludge becomes gel-like, includes more alkali and does not sediment well. Under particularly unfavorable conditions, the regeneration of the caustic sludge through SiO ₂ enrichment is completely impossible.

Many of the known methods for silica removal from black liquor (German Patents 522,730 and 523,485; Hungarian patent specification 143 291) relate to the introduction of carbon dioxide, within certain p _H values and a consequent precipitation of silicic acid in gel form.

However, these procedures all suffer from the difficulties which one encounters when separating the gel-like silica sludge, and therefore have also not yet found its way into practice.

The fine colloid nature of the separation makes filtrations almost impossible, even one takes without Aids carried out simple sedimentation too much time to complete, quite apart from the fact that the small difference between lye and gel in terms of density due to the use of centrifugal forces based separation seems almost hopeless. So will help remove this obstacle the seemingly complicated pressure heating of the precipitates in order to convert them into an easily filterable one Form proposed (German patent specification 522 730).

Another decilification process (USA patent process for the removal of silica from silicate-rich,

alkaline pulp waste liquors, especially in continuous operation

Applicant:

Institute for Pulp and Paper, Heidenau (Sa.), Pirnaer Str. 37

Horst Schmidt, Dresden, has been named as the inventor

script 2 628 155) uses quicklime in excess, namely in amounts of 25 to 50 g CaO per liter of waste liquor, from which it can be seen that this process is also not due to excessive lime requirement can completely satisfy.

The use of other sparingly or insoluble lime compounds such as Ca SO ₄ , Ca SO ₃ and CaS is known, but requires too large amounts of precipitant due to the insufficient dissociation of such compounds and the above-mentioned decilification with CaO. The way out of using soluble Ca compounds, for example CaCl ₂ , instead of sparingly soluble Ca compounds fails because these compounds not only with the silica, but above all with the lignin components of the waste liquor, which are present in large quantities, with the formation of fine colloids , difficult-to-filter sludges react.

The extension of the series of desiccating compounds (British patents 381697 and 393 962) and extension to freshly precipitated water-insoluble oxides and hydroxides of magnesium, Zinc and aluminum do not bring the desired success. The restriction also applies to these substances that because of their insufficient dissociation they decilitate only very poorly.

It has also become known (French patent specification 1 064230), silica from hot silicate solutions with carbonic acid in the presence of soluble ones To precipitate alkali chlorides or with the former alone in order to obtain a silica pigment which is rendered alkali-free by treatment with mineral acids, whereby the alkali can also be exchanged by reacting with magnesium chloride.- With this However, the method is not intended to remove silica from pulp waste liquor, but only to

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trium exchange or the removal of the sodium ion from the silica pigment previously precipitated with carbonic acid.

In addition to starch and glue products, MgCl _{2 has} also been used to clarify green liquor (JPCasey: "Pulp and Paper" 1952). In contrast to black liquor, green liquor does not contain any organic substances, but large amounts of inorganic salts such as NaOH, Na ₂ S and Na ₂ CO ₃ . _{If MgCl 2 is} added to this lye, then Mg (OH) _{2 is formed} which, when it settles, entrains suspended particles such as coal, lime and silicates. So this is a physical sedimentation process and no precipitation of dissolved silica, which does not occur in any significant quantities in green liquor.

The invention now opens up a new way for the continuous removal of silica from silicate-rich, alkaline pulp waste liquors and is essentially characterized in that the 80 to Black liquor has a temperature of 90 ° C, and easily soluble magnesium salts such as magnesium chloride or sulfate are correspondingly hot the lye composition and the amount of silica to be removed and the some Time with the precipitant well mixed liquor a settling tank, preferably a clarifier, its upper outlet made of low-silica lye and the lower outlet of silicic acid sludge, fed and the latter via any separation device, e.g. B. suction cell filter, from the excess caustic is separated with the least possible use of washing water.

The experimental research that preceded the invention resulted in the unpredictable finding that magnesium salts react primarily with the sodium silicate and not with the organic substances, which are also present in large quantities, as is the case with the use of soluble Ca salts. The _pH value of the black liquor undergoes through the additions of Mg salts only a small degradation, and therefore also the risk of precipitation of organic liquor ingredients in contrast to the carbonic acid precipitation is much less.

The silica deposits achieved are considerable in comparison to those obtainable with carbonic acid more favorable properties; they sediment much faster, are easier to filter and are technologically significantly easier to obtain, so that actually only through the present invention a continuously working, practicable desilification has become possible.

Execution s example

To 500 ml of black liquor of a reed on circuit containing 5.6 g / l of SiO ₂ and 118.8 g / l GesamtfeststofEe, at 80 g to 90 ⁰ C 0.61 MgSO ₄ in the form of lO ^o / cent solution while stirring gently added dropwise. After thorough mixing, the sample is filled into a graduated, funnel-shaped measuring glass and left to sediment. After 10 minutes, ₂ slush have separated out from the liquor 150 ml SiO, which contained 1.425 g of SiO _2, which corresponds to a Entkieselungseffekt of 2.85 g / l Si O ₂ corresponds. Expressed as SiO ₂ / MgO, the result is a value of 4.67.

The liquid standing above the precipitate, 51% freed from silica, could easily pass through Decantation and filtration are separated from the silica.

In contrast to this procedure, if CO _{2 is introduced} into 500 ml of black liquor in the heat up to a Pj ₁ value of about 9.9, which corresponds to the prior art, then not the slightest sediment has formed after 10 minutes, Instead, the entire volume of the lye has taken on the character of a gel and can therefore not be filtered.

After 35 minutes, the horn has reduced its volume to 400 ml, and only after 6 hours is it Settling effect of the first described sample achieved. In

ίο comprehensive test series could also the chemical The principle of Mg-Salt-Entkiesekmg clarified will.

In view of the fact that there is a very complex system in the black liquor which, in addition to various types of wood degradation products, also contains residual amounts of the digestion chemicals (NaOH, Na ₂ S and Na ₂ CO ₃ ) and the alkali salts of organic acids, it was initially assumed that primarily forms an insoluble magnesium compound (hydroxide, sulfide, carbonate), which only secondarily binds SiO ₂ .

The examination of the silicification effect of freshly precipitated magnesium hydroxide and carbonate showed but their almost complete inactivity compared to the

as soluble magnesium salt. Only when freshly precipitated magnesium silicate, which roughly _{corresponded to the composition MgO-SiO 2} , was used, was the effect of the soluble magnesium salt achieved.

The principle of Mg-salt decilification is to be briefly formulated as follows:

Soluble magnesium salts react when introduced into black liquor primarily with the sodium silicate to MgO-SiO _2, immediately in ₂ amounts MgO · (SiO _{2) 3} _ ₅ reacts in the presence of further SiO.

Too large a residual amount of free NaOH in the black liquor has a disruptive effect, since the Mg salt converts _{the hydroxide into Mg (OH) 2 before it reacts with the sodium silicate.} The more NaOH is present, the more Mg salt is required.

On the other hand, Na ₂ CO ₃ does not have an inhibiting effect, since it only _{ranks after Na 2} O · Si O ₂ in the reaction sequence. It therefore makes sense to convert _{disruptive NaOH into Na 2} CO ₃ by introducing CO _2.

The amount of Mg added is completely found in the precipitate.

In general, it should also be said that the decilification effect given in the example can be improved by increasing the addition of Mg salt, e.g. B. in such a way that an increase in the _{addition of MgS O 4} from 1.22 to 1.83 g / l an improvement in the silica removal effect from 2.85 to 3.55 g / l and .somit 63% SiO ₂ -Eliminate causes.

The increase in the amount of precipitated silica with increasing addition of Mg salt is approximately linear in these ranges and can be characterized by a SiO ₂ / Mg O quotient between 4.5 and 5.0.

It is advisable, when separating the deposited silica by filtration, to remove it adsorbed lye constituents to accomplish the necessary washing with as little water as possible, since the Silica deposition has considerable solubility in water. ""

So were z. B. in the context of a laboratory test, when 150 ml of water were used, 820 mg of SiO _{2 were} obtained, while washing with 500 ml of only 470 mg of SiO ₂ remained; So 350 ml of water have dissolved _{350 mg of SiO 2.}

However, this is not a specific disadvantage of the method, but a general property of

Si (^ precipitates and meets the carbonic acid process as well.

Another fact that deserves to be emphasized is that as the SiO ₂ content of the black liquor increases, the amount of silica that can be precipitated also increases. It was found that with a content of 2.7 g SiCyi by IMgO 2.75 SiO _{2 are} precipitated, while with an alkali with 5.6 g / l SiO ₂ the ratio was 1: 4.78. So the question of the optimal decilification effect can only be answered if the lye composition is known.

A significant loss of active alkali cannot be ascertained with the additives used, likewise larger losses of organic substances are not to be feared. A quick wash of the Sludge during the separation will easily remove adhering residues of both substances.

Depending on the economic situation, efforts will be made to regenerate the magnesium used and to practically implement a magnesium cycle. The easiest way to do this is to introduce the extracted silica sludge into hot, dilute mineral acid until it has completely reacted. As a result, the Mg content of the sludge is dissolved and can be used again immediately after filtration for decilification, while at the same time a fairly pure silicon dioxide, which can be used for a variety of purposes, is obtained. It is advisable to work _{preferably with MgSO 4 and consequently to regenerate with H 2} SO ₄ , since Mg Cl ₂ can lead to complications in the melt due to the NaCl formed during reaction with the Na ₂ SiO _3.

In the course of the investigations, a further important variant of the process has emerged, namely in that it has been found that the additions of soluble Mg salts according to the invention - naturally in smaller amounts - give the silica gels deposited in the above-mentioned carbonic acid process properties that give a make simple sedimentation processes possible. To achieve this, is added to the hot black liquor prior to the CO ₂ -Emleitung stirring small amounts of easily soluble magnesium compounds such as MgCl ₂ and Mg SO ₄ to, and then operates only while stirring the known acidification with CO ₂ to p _H - Values of about 9.9.

The success is expressed in the fact that the gel, which is without additives, as already mentioned in the embodiment was, sedimented only in 6 hours from 500 to 150 ml, this effect is now already after 15 minutes reached.

This means that a continuously operating sedimentation process is made possible.

To achieve this sedimentation-promoting effect, additions of 0.5 to 0.6 g of MgS O ₄ or MgCl _{2 are} required per liter of black liquor, i.e. considerably less than would be necessary for complete decilification. It must be ensured that after the addition of the sedimentation- _{promoting agent, the introduction of CO 2 is} carried out with thorough mixing, so that good inclusion of the active substances in the flocculating gel is ensured.

The effect of the described, inventive sedimentation-promoting additives can with some probability be explained by assuming that that the additions or the substances resulting from them when they are included in the subsequent precipitating gel the physico-chemical nature of the same as a result of partial polysilicate formation and becoming effective Change adsorptive forces so advantageously that the sedimentation is many times over is improved.

The technology of the method according to the invention is very simple and requires little equipment expenditure.

The method according to the invention is explained in more detail in the drawing in two exemplary embodiments.

ίο The description is based on a sulphate cellulose plant with a daily production of 120 t and accordingly an accumulation of 1000 m ^{3 of} black liquor, the SiO ₂ content of which is to be reduced from 5.8 to 2.5 g / l.

According to FIG. 1, the 80 to 90 ° C. hot black liquor 1 is continuously mixed with MgS O ₄ solution 3 in a mixing vessel 2. The size of the mixing container should be such that a residence time of 1 to 2 minutes is guaranteed, that is about 2 m ³ .

ao The daily Mg SO ₄ consumption would amount to 1.25 t Mg SO _{4 on the basis above.}

The lye, thoroughly mixed with the precipitating agent, flows evenly to the sedimentation funnel 4 and has the opportunity to sediment ^{there after a residence time of 20 to 30 minutes (approximately 20 to 25 m 3 capacity).}

Low-silica black liquor constantly flows over the funnel edge 5, while silica-rich sludge is continuously withdrawn from the funnel tip 6 and removed from excess lye, e.g. B. is separated by filtration in the mud filter 7 with little water washing. The separated sludge 8 is optionally treated for regeneration by one of the processes mentioned for the purpose of recovering a re-usable magnesium compound or a versatile silicic acid. The SiO ₂ -free lye 9 is fed to the evaporators.

According to Fig. 2, 90 ° C hot black liquor 1 and the inventive sedimentation-promoting additive 3 together flow through a mixing channel 10 and enter a reactor 11, into which CO ₂ -containing gases 12 are continuously injected, with thorough mixing of all the material and below maintaining a lying 9.7 to 10.0 p _H -value.

For a 60 to 70% decilification effect, 0.5 to 0.6 t of MgCl ₂ or MgSO _{4 would result} as sedimentation-accelerating additives required daily with the aforementioned amounts of caustic.

The residence time of the mixture in the reactor should expediently be 2 to 3 minutes, which is why it should have a capacity of 2.5 to 3 m ³ in the above example. , Brought to the desired _pH value mixture is continuously withdrawn and a sedimentation funnel 5, respectively, of itself be designed for 30 minutes residence time and, consequently, about 25 to grasp tn. ³ From the funnel, de-silicified liquor 5 flows continuously in the manner already described, while 200 to 300 m ^{3 of} silica sludge 8 are drawn off daily from the funnel tip 6 and sucked off through a filter 7 with as little washing water as possible at the turn. The SiO ₂ -free lye 9 is fed to the evaporators.

The advantage of the method according to the invention can be seen in particular in the fact that the deposition of the greater part of the silica takes place in easily sedimentable form, the inventive Additive or that arises from it in the lye

The existing polysilicate gives the silica gel subsequently precipitated by carbonic acid such physico-chemical properties that its sedimentation capacity, in relation to time, increases many times over and is therefore an unprecedented one simple continuous descaling enables.

Claims (3)

Patent claims: 1. Process for removing silica from alkaline pulp waste liquors rich in silicate by precipitation of the silicates with magnesium compounds, characterized in that the silicates be precipitated in the hot black liquor with easily soluble magnesium compounds and the deposited precipitate is removed. 2. Modification of the method according to claim 1 for the continuous removal of the silica from the waste liquors, characterized in that a smaller amount of magnesium compounds for precipitation than required for complete decilification used and then in a known manner carbon dioxide until coagulation the silica is introduced into the waste liquor. 3. The method according to claim 1 and 2, characterized in that the flocculated and separated Precipitation sludge is treated with a hot, dilute mineral acid and thereby the Magnesium compounds are recovered in the cycle. Considered publications:
German patent application A18276 IVa / 55 b made known on January 27, 1955); French patent specification No. 1 029 169, 230;
British Patent Nos. 381 697, 393 962. 1 sheet of drawings © 909 757/148 2.60