FI66033B - FOERFARANDE FOER FRAMSTAELLNING AV KOKLUT - Google Patents

Description

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Method for making cooking liquor - Förfarande för framställning av koklut

The present invention relates to a process for preparing an alkaline cooking liquor, in which a waste broth containing cooking borate is burned into a melt which is treated for the reuse of chemicals.

5 The incineration of time-consuming cooking liquors to recover chemicals and heat yields a melt, one of the main components of which is sodium carbonate. In the case of sulphate soup waste liquor, i.e. black liquor, sodium sulphide is also formed. However, in most cases, the carbonate is not alkaline enough to dissolve the lignin in the wood, causing the carbonate to be converted to a hydroxide. This process is called causticization. Causticization can be performed as auto-causticization, as described in Finnish publication 53141.

In autocastrification, sodium borate is added to the Na 2 HBO 2 digester-15 chemical circuit. Alternatively, other suitable salts may be used, as disclosed in said publication. The causticization then takes place as follows: 2NaH2B03 + Na2C03 · * 2Na2HB03 + CC> 2 + HjO Formula 1

In combustion, CO2 and H2O are released and Na2HBO3 remains in the melt, which is dissolved and can be used in cooking. Na2HBO3 has been found to correspond to NaOH in cooking in a molar ratio of approximately 1: 1. The melt produced in the incineration, which contains the chemicals Na, B and possibly S, is dissolved in water, and the resulting cooking solution is used as such to delignify the wood. This cooking solution 25 generates, in cooking with wood, a waste liquor which is evaporated and incinerated, i.e. The good side of the method is its simplicity, but it also has some weaknesses.

When sulfate cooking is performed and borate is added to the chemical circuit to effect autocastification in a recovery boiler, the amount of salt circulating in the cooking boiler increases. Borate soup contains one mole of NaH2BO3 per mole of NaOrg, which is useless for cooking ______ - τ ~ 2 6 3 0 3 3, but which is needed to causticize the sodium carbonate produced by combustion (see Formula 1). The borate salt present in the cooking slows down the cooking and to compensate for the slowness of the cooking, either the alkali portion must be increased or the cooking temperature 5 must be raised. It is also suspected that the yield will decrease. Upon evaporation of the waste solution, the boiling point increases and the viscosity is higher than with normal sulphate waste liquor. In addition, borate waste liquor already gels in the dry matter by 45-55%, making further evaporation difficult, while normal sulphate waste liquor can be evaporated to more than 60% dry matter without gelling problems.

It is an object of the present invention to obviate or partially eliminate the difficulties caused by unnecessary salt loading in the borate process. The salt load is reduced by removing part of the borate 15 either by incineration in the melt, by extraction or partial dissolution of the melt during dissolution, or by crystallization of the borate from the cooking liquor. A diagram of the process according to the invention is shown in Figure 1.

Example 20 When borate was added to the white liquor of the sulfate soup to effect autocastification, the borate content of the cooking liquor could be reduced by 70%. In addition to Na 2 S, the melt contained sodium and boron in a molar ratio of Na: B = 2: 1. All Na2S was dissolved in water at different temperatures, so that part of the borate also dissolved.

25 The concentration of borate in the residue increases as the temperature decreases.

When the extraction is carried out with 45 ° C water, the residue is almost pure NaBCl 2 x 1 O and the molar ratio of the residue is Na: B = 1: 1. The melt, solution and residue compositions shown in Table 1 have been obtained by extracting the melt with water at 45 ° C.

mol B mol Na mol S Table 1

Sula 2.47 6.0 0.53

Solution 0.70 4.2-4.3 0.50 Residue 1.7-1.8 1.7-1.8 0.03

Extraction at 45 ° C with water 30 The residue is almost pure NaBO 2 x 1 → 0, so that the broth contains the same amount of act. alkali as if the whole melt had been dissolved, because:

Na 2 HPO 3 = NaBO 2 + NaOH Formula 2

In the process according to the invention (Fig. 1), a soup waste liquor is incinerated in a recovery boiler, to which borate is added before incineration or directly to incineration. In order to achieve a high degree of causticization, the addition of borate should be such that the ratio of causticized Na2CO3 to Na2B £ 2. The melt produced in the combustion is extracted so that most of the cooking chemicals remain ). The resulting solution is used in cooking as such or possibly modified. The resulting solution should be as concentrated as possible to reduce the steam consumption of the soup.

The residue from the extraction, which contains relatively more borate than the cooking solution, is recycled back to incineration.

15 Thus, a closed borate cycle is obtained and only the amounts required for process losses are added to the new borate.

Borate can also be reduced from the cooking solution by crystallizing borate from the solution obtained when the entire amount of melt is dissolved. Other methods for separating borate can be applied, such as partial dissolution. The borate-containing residue circulating directly to incineration can be treated in various ways before incineration, for example by washing or drying, to improve chemical or thermal economy.

The amount of heat contained in the melt generated during combustion may be too high in relation to the amount of water required for extraction. In this case, the melt must be either partially or completely dissolved, unless it can be cooled in any other way. The sparingly soluble borate moiety can be crystallized from the resulting solution, for example, by removing water from the solution by evaporating or cooling the solution, thereby reducing the solubility of the borate. The resulting borate crystal mass is returned to incineration and a concentrated and / or cooled solution containing the majority of the cooking chemicals is introduced into the soup to delignify the wood.

4,60033

The process according to the invention for preparing a cooking solution can be applied to several cooking methods in which one of the active chemicals is NaOH. In the most commonly used cooking process, the sulfate process, the active chemicals are Na 2 S and NaOH, and especially in this case, as well as in the soda process, the process of the invention can be used to causticize ^ 2 ^ Na to NaOH. The broth may be used as such in cooking or, after evaporation, concentrated or otherwise treated. An additional chemical, for example anthraquinone, may also be added to the cooking solution before cooking.

An additional chemical can also be removed from the residue before returning the residue to incineration. Such a chemical may be, for example, NaCl or some other undesirable component of the chemical cycle.

NaCl is a chemical that is extra and harmful to the cooking chemical cycle, which can be circulated with, for example, wood or bleaching effluents. There are many methods to remove it. In the process of this invention, NaCl can be removed by washing the borate residue with water because NaCl is less soluble under the conditions of cooking chemicals Na 2 S and NaOH, but more readily soluble than borate.

The waste liquor from the borate soup can thus be burned in such a way that the active chemicals in the soup are formed without all the borate required for autocust-25 sound being recycled back into the soup. The amount of salt going to cooking and evaporation is kept at a lower level than in normal borate soup. Table 2 shows how, in the case of sulphate, the amount of salt in the extraction method according to Table 1 relates to the amount of salt in the borate soup 30 and the sulphate soup. Extraction of the melt is also advantageous because it results in a more concentrated cooking solution, which in turn reduces the steam consumption in cooking and evaporation.

The advantages of this new method over normal autocusting to causticize Na 2 CO 3 are obvious. Cooking and evaporation of li 6 ό 0 3 3 are as easy as with normal cooking processes. Borate losses are reduced because the borate cycle is shorter than the cooking chemical cycle. The cooking liquor is more concentrated than, for example, in a normal sulphate process.

Table 2:

Quantities of cooking chemicals per tonne of pulp in cooking and evaporation

Sulphate Borate Borate

Normal Extraction 45 ° C

(NaOH) 2 kmol / t 5.0 - 3.25

Na2HBO3 - 10.0 3.50

Na2S 2.0 2.0 2.00

Total kg / t 560 1220 790 o 3 3

Cooking liquor volumes: 4.0 m / h 4.0 m / t 2.0 m / h

Claims (8)

A process for preparing cooking liquor, in which a waste liquor containing cooking borate is incinerated into a melt which is treated for chemical reuse, characterized in that most of the borate is separated from the melt by forming a borate-containing solid phase and a liquid phase containing or, if necessary, concentrated in the broth and the solid phase is returned to combustion. Process according to Claim 1, characterized in that the solid phase is separated by extraction. Process according to Claim 2, characterized in that the temperature of the water used in the extraction is about 45 ° C. Process according to Claim 1, characterized in that the solid phase is separated by crystallization. Process according to Claim 4, characterized in that most of the melt is dissolved and the resulting solution is cooled to separate the solid phase by crystallization. Process according to Claim 5, characterized in that the solution is cooled to 45 ° C. A method according to claim 1, characterized in that chemicals are removed from the solid phase before it is returned to incineration. Process according to Claim 7, characterized in that the chemical to be removed is NaCl.