FI73012B - FOERFARANDE FOER KOKNING AV SULFITCELLULOSA. - Google Patents

Description

1 73012

Method for cooking sulphite pulp

The invention relates to a process for producing sulphite pulp.

According to the invention, a cooking solution so concentrated is absorbed into the chips at the beginning of cooking that more than 35 5 of the total liquid can be depleted, but leaving such a solution that it can be recirculated through the calorizer to , 3 to 3. After depletion, the weight ratio of cooking solution to wood is in the range 2.6: 1 to 2.2: 1, preferably 2.4: 1. The solution is thus left in the digester only to the extent that the digestion solution can be circulated through the calorizer and thus the temperature of the digestion can be raised and adjusted by means of the circulating acid, and no direct steam addition as in the so-called höyrvfaasikeitossa. Thus, the process according to the invention 20 can be called a semi-vapor phase process, since the chips are only partially covered by the solution.

The method according to the invention requires that the chemical content of the cooking acid fed to the beginning of the cooking is so high that the amount of sulfur dioxide and base remaining in the kitchen after the impregnation step and the deficiency is sufficiently large for the actual cooking operation. This is achieved when the basic amount of sulfuric acid binds at least 13 g / l of sulfur dioxide and the total amount of sulfur dioxide is more than 80 g / l, ie the proportion of sulfur-bound sulfur dioxide is more than 16% of its total content. When the pressure is kept as high as possible during the actual cooking, i.e. the degassing is minimized to match the pressure resistance of the digester, the sulfur dioxide content remains sufficient.

The cooking acid is absorbed into the chips at a temperature of 80 to 110 ° C and the deficiency takes place in the same temperature range.

The actual cooking is carried out at a higher temperature than the impregnation step, i.e. at 120-150 ° C.

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As the base, known sulfite circulating bases such as calcium, ammonium, magnesium or sodium sulfite or a mixture thereof can be used.

The purpose of the initial cooking part of the process according to the invention is mainly to absorb the cooking solution into the chips and not the actual sulphination and / or dissolution of the lignin, since it is desired to contain only a small amount of dissolved organic matter when returning to the acid system.

It is known that the sulfur dioxide content can be increased by introducing liquefied sulfur dioxide into the digester. The amount of base, in turn, is increased by increasing the so-called the concentration of tower meat by adding the carbonate, hydroxide or oxide. The base addition can also be made directly to the chip 15 fed to the digester as the corresponding base compounds.

Another important finding of the invention is that in order to allow a large deficiency in calcium sulphite soups, the calcium content of the acid can be increased to a sufficiently high level also by adding soluble calcium salts directly to the cooking acid. When, for example, calcium acetate is added, the calcium content of the cooking acid can thus also be raised to a sufficiently high level.

Other advantages of the method are: less energy is used to heat the 25 cooking acids, the effluent is about 4 percentage points more concentrated in the evaporator, ie less energy is required for evaporation, faster cooking, higher pulp yield and 30 pulp lighter, and the lignosulfonate of the effluent greater than the normal method.

Finnish Patent No. 35,726 discloses a process for preparing sulphite pulp by carrying out cooking 35 in two steps, in the first step 110-130 ° C

and in the second stage at a higher temperature, whereby after the first stage about 3 73012 of the volume of the cooking liquid is removed and the amount of liquid is transferred to the central calorizer tank. The process according to the invention does not use a central calorizer, the temperature of the impregnation stage is only 80-110 ° C, and in both the first 5 and second stages the heating is carried out by circulating cooking liquid, whereby less liquid is sufficient and heating fumes do not condense. than the result of a known method.

10 Finnish Patent No. 54,938 discloses a method for improving the thermal economy in the production of pulp. Based on the working examples, it can be concluded that: · it is a sulphate soup, whereas instead the soup according to the invention is a sulphite soup, so the methods are completely different. In addition, the known method uses a pressure blow technique to enhance the absorption.

Finnish patent application No. 812102 discloses another method for improving the thermal economy in the production of sulphate pulp. Like the Finnish Patent No. 54,938 discussed above, this method is completely different from the method according to the invention. To enhance the impregnation, a pressure treatment is performed at the beginning of the impregnation step, and the final step is steam phase cooking.

Swedish Patent No. 175,684 discloses a process for cooking pine chips in two steps, in which case sodium is used as the base (calcium is not visible at pH 4-7). In this method, after the first step, the cooking liquid is removed almost completely, and liquefied sulfur dioxide and / or an aqueous sulfur dioxide solution is added. 3 0 Examples

Batch cooking series were performed at both laboratory and factory scale, in which the composition and concentration of the starting acid and, above all, the amount of deficiency were changed. In factory experiments, the deficiency was increased from normal 25% to 35-35 to 41% and in laboratory measurements from 20% to 40-45% of the total fluid volume.

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Batch soups were also performed at the laboratory scale, to which, in addition to calcium oxide, calcium acetate was added to raise the amount of calcium to the level required by high deprivation.

5 The results are shown in Table I on page 4.

Explanations of Table I: 1) raising the temperature from 50 to 85 ° C for 2 hours "" from 85 to 130 ° C for 3.5 hours 2) of which 4.3 g of SO 2/1 bound corresponding amount of base 10 added as calcium acetate 3) of which 7.0 g of SC ^ / l of bound base added as calcium acetate 4) of which 4.3 g of SC ^ / l of bound base added as ammonium acetate 5) without acetate addition, i.e. a small amount of base 6) of which 4.7 g of SC ^ / l of bound corresponding base added as calcium acetate.

Claims (10)

1. Process for the production of sulfite cellulose, characterized in that the chips at the beginning of the boiling are soaked with such a strong boiling solution that the total amount of liquid can be reduced by more than 35%, leaving such a solution in the boiler, however, that may be circulated through a calorifier to directly increase the temperature of the final boil, and wherein the pH of said strong boiling solution is 1.3-3. 2. A process according to claim 1, characterized in that the weight ratio of the cooking solution to the wood is reduced by 2.6: 1 to 2.2: 1, or even by the reduction. lower, and countywide to 2.4: 1. 15 3. A process according to claim 1, characterized in that the sulfur dioxide content of the boiling solution is above 80 g / l and its base concentration is high, that the base binds at least 16% of the sulfur dioxide amount, i.e. the pH of the boiling solution is 1.3 - 3. 20 4. A process according to claim 1, characterized in that the boiling solution is absorbed at a temperature of 80 - 110 ° C and that the amount of liquid is reduced at the same temperature. 5. A process according to claim 1, characterized in that the boiling solution is concentrated by adding liquid sulfur dioxide to the digester. 6. A process according to claims 1-3, characterized in that the base concentration of the boiling solution is increased by adding the boiling solution or the chip with 30 base compounds. 7. A process according to claims 1-3, characterized in that the base concentration of the boiling solution is increased by adding soluble calcium salts. 8. A process according to claim 7, characterized in that the boiling solution or chips are provided with calcium acetate.