NO792311L - PROCEDURE FOR THE USE OF SULFIT WASTE FROM INDUSTRIAL CELLULOSE PREPARATION - Google Patents

Abstract

Process for the utilization of sulphite waste air from industrial cellulose production.

Description

Utilization or environmentally friendly removal of waste liquor from sulphite cellulose factories, which per tonnes of cellulose contain

so to speak, the same amount of organic matter in 8-10 m 3 of leachate still constitutes

a problem that has not been satisfactorily resolved. A significant reason for this is that during the usual boiling of the sulphite liquor after depressurization in the pressure vessels, S02 is admittedly avoided, while sulphite remains in the solution and cannot be completely separated even by precipitation. This applies not only to calcium sulphite leachate, but also to alkali and ammonium sulphite leachate as well as to the magnesium sulphite leachate formed by the magnesium sulphite process preferred today. Especially in the latter method where not only SC^ but also magnesium oxide is recovered, for the final removal of the sulphite liquor, the sulphite liquor is increasingly being burned, whereby part of the heat necessary for the boiling is obtained.

The incineration of the waste liquor, which contains high-quality natural products in the form of ligninsulfonic acids or their salts and in the form of sugars, is in itself an unsatisfactory utilization method. Other methods of utilization have not gained acceptance up to now, even though they have been proposed in large numbers, and pollution of waterways due to sulphite leachate is no longer an option in most countries due to current environmental protection legislation. Not only the methods already proposed up to now for utilization of the carbohydrate and lignin portion of the sulphite liquor, but also the relatively imperfect precautions in connection with combustion, suffer from the incomplete expulsion of SC^ by the known methods, especially however expulsion of the sulphite.

Combustion of calcium sulphite leachate thereby gives, in addition to SC>2, also the flue gas SO3, which forms. CaSO^ and together with other axis constituents also unwanted waste. The magnesium bisulphite process must contend with similar problems.

According to the invention, it is contrary to this and for

to solve the described deficiencies, provided that phosphoric acid or substances which form phosphoric acid upon hydrolysis are added to the sulphite liquor, after which the mixture is boiled until complete removal of SC>2 and especially also from the sulphite, after which the phosphoric acid is recovered.

In and of itself, the possibility of expelling SC^ from the sulphite liquor, especially also from the sulphite, by heating with acids and especially also with phosphoric acid, was already long ago described in a work by Kerp and Wohler ("Arb. d. Kaiserl. Gesundheitsamtes", 32nd volume, 1909, pages 129 and 130). For: technical purposes, however, this method at the time (compare German patent no. 236,035) was expressly stated to be unusable as it

found it difficult to recover the relatively large amounts of added acid. Thus, for the long time since this was first mentioned, boiling the sulphite liquor with phosphoric acid,

so to speak, not used at all, even though it has essential properties for the reaction used.

Phosphoric acid is a medium strong acid which, in concentrated solution, due to a content of diphosphoric acid, exhibits the pH value of an IN strong acid. It is not volatile and can therefore perfectly drive out equally strong volatile acids when boiling. It can be mixed with water and is therefore obtainable in any concentration and can be triggered by any solid system with water.

The acid is chemically very stable and acts neither oxidizing nor reducing at temperatures up to over 300°C.

It forms poorly soluble calcium salts and can be removed completely from aqueous solutions with the help of these.

It has a water-attracting effect in higher concentrations and therefore has a polymerizing or condensing on water-splitting organic substances. By adding phosphoric acid, it is achieved that the medium-strength, but not volatile phosphoric acid drives away the volatile sulfuric acid.

The reaction that occurs when sulphite leachate is boiled

at a pH value below 3.0, is described as follows:

However, SC>2 is not the only volatile substance in the sulphite liquor, there is also acetic acid and formic acid as well as furfural and cymol, whereby these substances are distinctly disadvantageous for the further utilization of the sulphite liquor, especially fermentation, but also for use as a precursor, as garvestbff aggregate, as liquefier for concrete, etc. On the other hand, when seen in isolation, they are valuable products. Furthermore, it is essential that e.g. the pentoses present in the sulphite liquor are converted to furfural with phosphoric acid during the boiling, so that the yield of these valuable starting materials for plastics and chemical syntheses is significantly increased in relation to the usual methods for the utilization of sulphite liquor.

The higher polymer sugars also present in the sulphite liquor are converted by boiling with phosphoric acid into simple sugars, whereby the fermentable proportion of the substances in the sulphite liquor is raised, and also makes its use as cattle fat more valuable. Furthermore, boiling the sulphite liquor with phosphoric acid is a good preparation for use as a tanning agent, as substances are obtained whose tanning optimum lies at a desirable lower pH value by splitting off sulphonic acid groups and exposing the phenolic hydroxyl groups.

It has been shown that the passage of gas, it is possible here to use ordinary air, improves the removal of SO 2 and other volatile products, especially acetic acid, formic acid, furfural and cymol. Boiling of the sulphite liquor is continued until no more SC^ escapes.

The resulting steam is condensed - whereby the main proportion of SC>2 as well as acetic acid and formic acid is found in the condensate. The non-condensed gases are captured as they e.g. is passed through milk of lime, calcium sulphite solution or a CaCO^ slurry. A remaining residue is finally taken up on activated carbon and removed from this by heating, e.g. by water vapor. Excreted SC>2, brought into Ca (OH) 2_solution, is returned as calcium sulphite or calcium hydrogen sulphite for the cellulose manufacturing process where sulphite liquor is produced.

The present invention offers two options for recycling the added phosphoric acid in order to be able to return it to the circuit. According to the first method, it is ensured that the phosphoric acid is immediately transferred to insoluble phosphate and then separated. For this purpose, Ca(0H)2 is added, whereby the pH value must be raised to approx. 7.0, which causes a transformation of the soluble Ca(H2P04)2 into the sparingly soluble CaHPO^ and finally into Ca^(PO^)2 and into apatite. The reaction is conveniently carried out at temperatures above 40°C, in particular because CaHP0 ..2H„0 here is transferred to CaHOP.+2H„0. Because this is not

4 2 4 2

special precautions are necessary as the sulphite lye must still be heated to higher temperatures when boiling. The simultaneous excretion of iron salt facilitates the use of the rés solution for tanning purposes.

In order to accelerate the described reaction, it is appropriate to add Ca(OH)2 in excess, i.e. to initially raise the pH value to significantly above 7.0, and after the formation of the desired poorly soluble salts, to neutralize the solution by bubbling through of CC^.

The in and of itself imminent recovery of phosphoric acid in the form of insoluble calcium phosphate by adding limestone instead of Ca(OH)2, would lead to the avoidance of CO,,, which in turn would bring the lignin sulphonic acid to an uncontrollable foaming. Furthermore, when limestone is added, the pH value only rises to approx. 4.5, while to obtain an essentially insoluble phosphate, approx. the neutral point.

The essential main reaction described here in the case of calcium sulphite leachate proceeds in an analogous manner with other types of sulphite leachate.

The further use of the obtained solution, which no longer contains sulphite and phosphate, can take place in different ways. If the construction of a plant for separate further processing of the carbohydrates is not justified from an economic point of view, the entire solution is evaporated and e.g. used as cattle feed, or burned for heat. In particular, one naturally thinks here of the heat that must be provided for boiling the sulphite liquor for the production of cellulose from wood.

If, however, the amount of rising carbohydrates appears

to make the combustion uneconomical, the solution is subjected to a fermentation process whereby alcohol or yeast. After separation of these products, the remaining solution is evaporated, whereby solid ligninsulfonic acids or salts thereof. Vanillin, solvents such as products of hydration as well as tanning substances, further adhesives, binders in ceramics and metallurgy as well as cement aggregates can be produced from it, the products can be used in road constructions, for weed control and further as briquette pitch etc., whereby the solubility of all salts of the lignin sulphonic acids keep the application within certain limits, and above all limit the deposition of the products. For the most part, however, this end product would be incinerated again if there was no simple possibility to transfer it to an insoluble form. However, the second method for recovering the phosphoric acid provided within the scope of the invention offers just such an opportunity.

Namely, it has been found that upon further heating

of the sulphite liquor, which has been freed from volatile compounds by means of phosphoric acid to temperatures above the evaporation temperature, polyreactions of lignin sulphonic acid and the carbohydrates take place, whereby these are converted into suitable solids that can be used. The combustion of a very complicated natural substance such as lignin, which in nature forms the basis for the further life of the forest,

can thus be avoided. During evaporation and heating occurs depending on the amount of phosphoric acid in relation to the solids content as well as depending on the heating duration, speed and temperature and also depending on the starting material, after absorption

with water, gel-like to hard products whereby the latter completely retain the organic matter and are furthermore water-insoluble, while the former may also contain dissolved substances. According to this, the applicability of the products is very different. The gel-like ones can serve as a nutrient base

for biotechnical processes (fermentation, etc.) after washing out and neutralizing residual phosphoric acid with NH3, Ca(OH)2, CaC03, K2C03, etc. The semi-solid and swellable can after

absorption with water, filtration of the main part of the phosphoric acid, neutralization of the remaining acid together with emery or limestone flour, serve as arable soil aggregate, while the hardened products

can be used as a cation exchanger, but also as an aggregate for plastics, rubber, asphalt and further as a water filter and in carbon dust boiler plants. The phosphoric acid used can mostly be washed out and recovered as such or in the form of soluble or insoluble salts. The organic substances remaining in solution next to the gel-like products, as far as these have not already been subjected to a biotechnical utilization, can be subjected to a new reaction when mixed with remaining phosphoric acid, i.e. also made completely insoluble.

The exact temperature level is not essential. Usually heated to 100-180°C and preferably 110-170°C. In most cases, temperatures between 120 and 160°C are preferred. Higher temperatures are chosen when you want to obtain insoluble products, however, if you choose swellable products or gels, you choose a lower temperature. Usually, you heat up immediately to the desired temperature and keep everything at this temperature, but you can also raise the temperature slowly and reach the maximum value only after a few hours, e.g. 5-10 hours. In some cases, the nature of heating depends on the individual behavior of the mixture, e.g. for the purpose of foaming.

The duration of the heating also depends on the circumstances of the individual cases and usually amounts to 1-20 hours depending on the height of the working temperature. The amount of added phosphoric acid or of the phosphoric acid-forming compound (in equivalent quantity) is usually between 6 and 120 ml (calculated as 85%) per 1 lye (with an average value of 10% solids content). Preferably, 12-40 ml/l lye is used.

The method according to the invention is suitable for sulift-waste liquor from all common types of wood, especially from beech and spruce. The procedure has been tested on sap from spruce from Pols, spruce

with pine from Raubling, beech from Pols, from Hallein and from Lenzing. Certain differences occur, in particular beech shows a somewhat higher phosphoric acid requirement and has a stronger tendency to form blistered-voluminous products. Lenzing used magnesium sulphite leachate from beech, which showed no significant differences in relation to the other beech leachates. As you know, these are not suitable for alcoholic fermentation, but for yeast production.

The method can be used in an analogous way for calcium, magnesium, alkali and ammonium sulphite lye, whereby there are only corresponding differences in the per se low content of polymers on the cation, and whereby the recovered phosphoric acid upon repeated use with the cation used each time gives itself an enrichment, since phosphates of different composition can be withdrawn from this circuit at appropriate distances depending on the solubility and hydrolysis ratio.

The procedure has, in relation to all methods used up to now for utilization or making sulphite waste water harmless not only has the advantage that no worthless waste is produced, but also that high temperatures and thus also corrosion problems can be avoided, as flue gas cleaning etc. can be avoided to a very large extent, as one does not usually need to exceed maximum temperatures of 180 °C.

A further explanation of the invention follows by means of the accompanying examples.

Example 1

100 parts by volume of sulphite leachate with 12% dry matter content is added to 1 part by volume of 85% phosphoric acid and heated while passing air to boiling until the volume has evaporated to 3/4. Instead of phosphoric acid, you can add an equivalent amount of a compound that forms phosphoric acid on hydrolysis, e.g. Ca(H 2 PO 4 ) 2 . The sulphite tablet can be obtained from all common types of wood, especially beech and spruce. As the addition of phosphoric acid increases the volatility, especially for substances with acidic properties, this is usually added at the beginning of heating and at least not too late. The addition of phosphoric acid can be done all at once, but you can also add the phosphoric acid in portions during the evaporation. S02 and other volatile products, as far as these are not obtained in the water vapor condensate, can be partly retained in alkaline solutions and the rest with the help of activated carbon. The remaining solution is neutralized with Ca(OH)2. In this way, an excess can be used which leads to an alkaline reaction. The mixture is then returned to the neutral point using CO 2 . To avoid the formation of hydrogen carbonate and instead achieve precipitation of CaCO^, the entire process takes place at boiling temperature. The resulting poorly soluble calcium phosphate is filtered off.

The filtrate consisting of soluble ligninsulfonic acid or salts thereof as well as carbohydrates, are either immediately or after completion of a fermentation, evaporated to a dry state and burned to obtain heat.

Example 2

1 liter of sulphite leachate with 12% solids content is mixed with 30 ml of 85% phosphoric acid and heated to boiling until no SC^mer escapes, whereby gases and vapors are captured using milk of lime, calcium sulphite solution or CaCO^ slurry and finally activated carbon . The heating takes place until the water is completely expelled, possibly by passing air or inert gas or by means of negative pressure until the temperature rises to 140°C, after which the temperature is kept at this temperature for 1 hour. After cooling, there is a blistered, brittle water-insoluble mass that can be easily crushed, e.g. to 1 mm grain size. From this, approximately 95% of the phosphoric acid is first washed out using water or sulphite leachate, an acid that is returned to the circuit. The remainder of the phosphoric acid is then washed with water, and this is recovered by precipitation with CaCO^ and Ca(OH)2 as calcium phosphate. The solid polymerization product exhibits the properties of an ion exchanger, but can also be used as a water filter, as an aggregate for plastics, rubber, asphalt, etc. and for carbon dust combustion. Example 3 1 liter of sulphite leachate with 10% dry matter content is added to 35 g of Ca (H^PO^)^.H20, and the whole is heated to boiling while the gases and vapors are collected as in ex. 1. After a few minutes, CaHPO^ falls out, the substance is isolated and used as such. The solution is further heated until, after complete evaporation, a temperature of 120°C is reached, after which this is maintained for 3 hours. After cooling, the whole is treated with water,

and a soil-like product is obtained next to a brown solution. This is filtered off and fed to a further batch.

The soil-like product, which after extraction contains approximately 6% of the added phosphoric acid, is mixed with emery in such a quantity that an almost neutrally reactive and now calcium phosphate-containing product is obtained, which is suitable for arable soil improvement.

Claims (10)

1. Procedure for using sulphite leachate from the industrial production of cellulose whereby the sulphite leachate is heated and SC>2 and other volatile organic substances are expelled, characterized in that phosphoric acid is added to the sulphite leachate or substances which on hydrolysis form phos acidify, that the mixture is boiled until complete removal of S02, especially from the sulphite, after which the phosphoric acid is recovered. 2. Method according to claim 1, characterized in that, during the boiling with phosphoric acid, gas and especially air is passed through the sulphite liquor to support the expulsion of SC>2 and other volatile products. 3. Method according to claim 1 or 2, characterized in that the steam produced by boiling with phosphoric acid is condensed, whereby the majority of SC^ as well as acetic acid and formic acid are in the condensate and that the remaining SC>2 and especially furfural and cymol are separated using of activated carbon from the non-condensable off-gas. 4. Method according to claim 3, characterized in that the separated SC^ is fed into a Ca(OH)2 solution or CaCO^ slurry and fed back to the process for the production of cellulose from which the sulphite board liquor originates, as calcium sulphite or calcium hydrogen sulphite. 5. Method according to claims 1-4, characterized in that for the recovery of phosphoric acid after the boiling and by adding Ca(OH)2 to the preferably still warm solution, insoluble calcium sulphate is formed which is separated, whereby the remaining solution is evaporated, e.g. by spray drying to a product that contains solid but soluble ligninsulphonic acids or salts thereof, whereby the solution is possibly subjected to a fermentation process before evaporation and the volatiles formed respectively insoluble fermentation products are separated in a manner known per se. 6. ■ Method according to claim 5, characterized in that Ca(OH)2 is added in excess, i.e. to an alkaline reaction, and then neutralized by introducing C02 - 7. Method according to claim 5 or 6, characterized in that the solid end product is burned to obtain heat. 8. Process according to any one of claims 1-4, characterized in that to recover the phosphoric acid, the sulphite liquor is completely steamed in during the boiling, and then heated to temperatures up to 180°C. whereby, depending on the amount of phosphoric acid, the heating duration, speed and temperature, swellable soft to hard water-insoluble products are formed which, after crushing, are treated with water and separated from the aqueous solution containing the phosphoric acid, whereby this is recovered and the converted substances are transferred to a for the final purpose suitable condition. 9. Method according to claim 8, characterized in that the heating is carried out to 110-170°C. 10. Method, particularly according to claims 8 or 9, characterized in that one per 1 liter of sulfite leachate (e.g. with 10% solids content) uses 6-120 ml, preferably 9-80 ml and most preferably 12-40 ml of phosphoric acid (calculated as 85% by weight), or an equivalent amount of a compound that forms phosphoric acid.