FI96119C - Method for acidifying the filtrate with NaHSO3 solution - Google Patents

Description

96119! 1 j]

Method for acidifying the filtrate with NaHSO 4:

The invention relates to a process for acidifying a salt.

5 In the manufacture of cellulose from wood raw material under fairly long conditions, often in the form of so-called cellulose. by the sulphate process in which NaOH and Na2S act as active cooking chemicals, the fatty acids and resin acids derived from the wood extractant are saponified to form a salt. The soap added by the aforementioned organic components likewise contains so-called organic extracts from wood extracts. neutral substances.

Conifers contain both fatty acids and resin acids.

Deciduous trees contain only fatty acids and additionally higher amounts of neutral substances.

It is known to convert soap into acids, for example by treatment with sulfuric acid. It is the predominant method in the pulp industry. The resulting mixture of fatty acids, resin acids and neutral substances is called crude tall oil.

When cooking conifers or blends of hardwoods and conifers Ϊ 20, the soap is easily distinguished at various stages of the treatment and evaporation of the black liquor. The smooth operation of the mill requires the collection and further treatment of the plant. "

As the hardwood content increases, the quality of crude tall oil obtained for further processing diminishes, since only 25 fatty acids and neutral substances become available.

Pure pine is often added to pure hardwood soup

working oil or soap to extract the pulp I

the mouth does not rise too high. There is also soap in the pure leaf soups. If this soap is acidified, it will no longer be usable for further processing by current methods. Such soap must be burned as such or converted into tall oil.

'The acid quality is the best indicator of tall oil quality. 7 Pure pine soup in Finland produces a product with an acidic pathway of 160 to 170. The hardwood soup yields an oil with an * <2 96119 acid number of about 120 to 130. An oil with an acid number of more than 145 is suitable for further processing.

Soap with too low an acid number must be burned. This is often done in a recovery boiler. The Na content of the filtrate is completely recovered. Sodium often causes too much difficulty in a honey oven. Burning in a recovery boiler, on the other hand, requires that the recovery boiler has 5 to 10% excess capacity, which is not usually the case. The boiler is very often the bottleneck in production. Mi-10, if the sodium content of the leach could be cheaply and sufficiently reduced, burning in a lime kiln would also be possible.

The air protection objectives to be achieved in the next few years will limit the release of gaseous sulfur compounds to 15 kg of S / t pulp. With current technology, the emissions from a semi-coniferous hardwood mill are 1 kg S / t pulp for processes outside the tall pine stove.

Sulfur comes into the chemical cycle from about 2 kg S / t pulp from various sources such as wood, water, lime kiln oil and oxygen bleach mag-20sulphate. Already this results in an excess of about 0.5 kg sulfur / tonne pulp. It should also be noted that the waste acid from chlorine dioxide plants contains 7 to 24 kg S / t pulp. Thus, the waste acid must be drained or an equivalent amount of sulfur must be removed by other means.

25 Due to the above disadvantages, it has thus been necessary to provide a sulfur-free tall oil manufacturing process in which the necessary acidification chemical is produced from the mill's chemical cycle so that tall oil production and refining can continue in the future.

30 U.S. Pat. No. 3,901,869 discloses a solution. Partial acidification with carbon dioxide and final acidification with sulfuric acid or waste acid from the chlorine dioxide plant are preferred. However, it has turned out that as a result of this procedure, the sulfur content of the chemical cycle is still too high and investment is rising.

-9 96119 3

The modern pulp mill has to collect and burn the strong and dilute sulfur-containing odor gases already produced for environmental * reasons. Incineration is usually done in a separate incinerator or in a 5 lime kiln. In both cases, dilute sulfur dioxide-containing gases are obtained, the sulfur of which is absorbed into NaOH. Alternatively, the SO2 required for post-acidification of the filtrate may be absorbed into fresh (so-called "make-up") NaOH and the remainder of the SO2 ab-10 sorbed into the oxidized white liquor used for the initial acidification of the filtrate. The SO2 required for post-acidification is prepared from the resulting NaHSO3 solution by absorbing into NaOH, whereupon NaHSO3 decomposes to SO2 and Na2SO3.

The NaHSO 3 solution obtained from the absorption is about 20%. per cent response. The sulfur content of this solution is about 2 to 3 kg S / t pulp. If the factory has so-called. In an LHT plant, where the viscosity of the black-back is reduced by heat treatment, the amount of sulfur-containing gases produced therein can be increased so that the total sulfur flow reaches up to 10 kg S / t pulp.

As has been shown in the past, sodium bisulfite solution does not fit back into the chemical cycle within the future environmental requirements.

It is therefore an object of the present invention to provide a method that overcomes the above disadvantages. This object is achieved by the use of sodium bisulfite solution obtained by absorbing SO2 in a 1 "alkaline sodium hydroxide solution of flue gases obtained from the combustion of pulp mills, or NaOH or oxidized white liquor instead of sulfuric acid or chlorine dioxide waste acid - * NaHSO3-containing mother liquor obtained therewith. The acidification is carried out in one or more steps. Generally, 50-100% of acidification can be performed with sodium 4 96119 bisulfite solution instead of sulfuric acid. The concentration of the NaHSO 3 solution used is normally about 10-30% depending on the particular absorber.

The NaHSO 3 acidification is normally carried out at 90-150 ° C and atmospheric pressure or mild overpressure. When acidification is carried out in several steps, the separation of the mother water between the steps is performed. Compared to sulfuric acidification, the phase separation time is shorter in NaHSO3 acidification. The acidification can be carried out either as a batch method 10 or continuously. Sodium bisulfite acidification in one step reduces the Na content and viscosity of the filtrate. This makes it possible to burn in a furnace or in a recovery boiler. The more steps acidification is performed, the more complete the conversion to tall oil.

It has been experimentally shown that in a three-step bisulfite acidification, about 95% of the sulfuric acid can be replaced by a bisulfite solution. For example, in a mill that uses 2.1 kg S / t of sulfuric acid, the amount of sulfur introduced into the chemical cycle of tall oil production will be reduced to 20 0.1 kg sulfur / t pulp, which will close the balance sheet as future sulfur emissions come into effect.

The soap can be converted into tall oil either in the sulphate mill or in a separate cooking plant. Advantages of the invention include that when performing pre-acidification with sodium bisulfite solution, the water content of the partially acidified intermediate can be reduced, which reduces transportation costs and use of sulfuric acid when the final acidification is performed centrally. This is done at the pulp mill without bringing in the previously necessary additional sulfur.

The invention is illustrated by the following examples.

Example 1: Single-phase NaHSO, acidification * v

The experiment used a factory soap to obtain tall oil with sulfuric acid. The acid number of tall oil is * 146. The starting oil content of tall oil was 55.1% by weight.

A

· 5 96119

353 g of salt and 447 g of 20% w / w NaHSO 3 solution were weighed. The mixture was heated in a sealed reactor at 95-105 ° C for 30 minutes. Thereafter, bubbles of the formed gas were allowed to escape. The mixture was poured into 5 separate beakers which were placed in a heating cabinet (95 ° C) for clarification.

After clarification, the intermediate and the bulk of the lignin phase were separated from the upper phase for final acidification.

The weight of the separated phase was 264 g.

The final acidification was performed with sulfuric acid to pH 3, which is in accordance with normal factory practice.

In this case, the consumption of sulfuric acid, calculated as 100%, was 32 g / kg of soup. When acidified completely with sulfuric acid, the consumption was 82 g / kg of soup. Thus, pre-acidification with sodium r 15 bisulfite solution saved 61% sulfuric acid.

The volume of pre-acidified filter was about 70% of the volume of the original filter. When 1000 kg of soup contains 551 kg of tall oil and 449 kg of water and black liquor, this proportion decreased by 300 kg.

Example 2: Three-step NaHSO 3 acidification.

This experiment was performed as a one-step experiment, but the upper phase was separated and re-acidified with fresh sodium bisulfite solution. Fertilizers used in the various steps

· * The weight percentages between the 25 van and the intermediate were as follows: Step 1: 129% Step 2: 148% - Step 3: 122% 30 * “The temperatures at the various steps were 95-125 ° C. The sulfuric acid consumption of the product obtained after step three was 6.6% compared to sulfuric acid. ΐί

Claims (5)

96119 A process for acidifying a filtrate formed in pulp cooking, characterized in that the acidification is carried out mainly with a sodium bisulphite solution obtained by absorbing sulfur dioxide from flue gases from pulp mill incineration in an alkaline sodium hydroxide or sodium hydroxide solution of the sulphite solution is sodium bisulphite-containing mother liquor obtained from the sulfuric acidification of tall oil and that only the final acidification is carried out with sulfuric acid. 2. Förfarande enligt patentkrav 1, k e n - 15 tecknat därav, att surgörningen utförs i ett eller! flera steg, varvid mellan Stegen utförs separering av moderlut. Process according to Claim 1, characterized in that the acidification is carried out in one or more stages, where the separation of the mother water is carried out between the steps. 3. Förfarande enligt patentkrav 1 eller 2, k-n - netecknat därav, att surgörningen utförs vid en '20 to 90 - 150 eC. Process according to claim 1 or 2, characterized in that the acidification is carried out at a temperature of 90 to 150 ° C. 4. Förfarande enligt Face to face patent application, patent application, att surgörningen utförs vid atmosfärstryck et vid et svagt övertryck. Process according to any one of the preceding claims, characterized in that the acidification is carried out at atmospheric pressure or at slight overpressure. 5. A process according to any one of the preceding claims, characterized in that the final conversion of tall oil salt mixture from sodium bisulfite acidification in oxidized white liquor to tall oil is carried out with concentrated sulfur dioxide obtained by absorbing the 5 % sulfur dioxide gas in fresh 30% sodium hydroxide solution to give a sodium bisulfite solution, which is then made by evaporation of concentrated sulfur oxide. f. j 96119 '1. Förfarande för surgörning av papaad vid kokning av Cellulosa, kännetecknat därav, att 5 surgörningen utförs interesudsakligen med en sodium bisulphite-lösning, genome att absorbera svaveldens förräser-erns. i en alkaline sodium hydroxide, sodium hydroxide solution, sodium hydroxide solution, hydrated sodium, bisulphite salt, modern sodium hydroxide solution, sodium hydroxide, sodium chloride, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, 5. Förfarande enligt face av föregäende patent- ··: 25 krav, kännetecknat därav, att den slutliga omvandlingen av en tallolje-köpblandning erhällen vid en sodium bisulfitsurgörning under användning av en oxiderad vitlut, utförs med concentrerad svaveld vid förbränning av luktgas er- ^ 30 hällen utspädd 1-5% svaveldioxidgas, i en färsk nat- L t J riumhydroxidlösning, save att en sodium bisulfitlösning erhälls, ur vilken sedan genom avdunstning framställs concentrerad svaveldioxid. »» T