JPS6029796B2 - Alkaline pulping or alkaline whitening method. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for regenerating pulping or bleaching chemicals from pulping or bleaching waste liquor.

When alkaline pulping waste is combusted to obtain chemicals and heat, one of the major products is sodium carbonate.

In the case of black liquor from kraft cooking, sodium sulfide is also formed. The product is passed through a recovery furnace and then dissolved in water to form a so-called green liquor. Generally, carbonates are not sufficient to alkaline pulpwood or similar fibrous materials to any suitable degree. This results in the conversion of carbonate to hydroxide. This process is called causticization and is carried out using metal hydroxide solutions, the corresponding carbonates of which have low water solubility. in fact,
Calcium hydroxide is used for causticization. In this case, in addition to soluble sodium hydroxide, sparingly soluble calcium carbonate (lime sludge) is produced, which is usually separated, heated until converted to calcium oxide (lime sludge reburning), and then dissolved in fresh calcium hydroxide. do. Since such causticization requires equipment and time, avoiding it represents significant savings to the pulp mill. The present invention seeks to eliminate such causticization due to the addition of chemicals and separation of created organisms. This can be accomplished through the use of chemicals other than those conventionally used in alkaline pulping processes. Since alkali is also required for pulp bleaching, conventional bleaching alkalis can be replaced with chemicals that can be regenerated by the present invention. Another advantage of cooking and bleaching with such chemicals is that a more uniform pH is obtained and therefore less carbohydrate degradation occurs. Furthermore, the chemicals can be used for alkaline treatment of pulps already formed, for example in the viscose process. Such liquids can also be produced according to the invention. The causticizing process according to the present invention will hereinafter be referred to as automatic causticizing.

This is applicable when certain salts of polybasic inorganic acids such as boric acid or orthophosphoric acid are used as pulping chemicals (or bleaching chemicals, usually delignification chemicals). After using such chemicals, the waste liquid is evaporated and burned, and the organic matter is then converted into mainly carbon dioxide and water, whereby a part of the carbon dioxide is converted into carbonate and a non-volatile residue (depending on the processing temperature, ash content or melt). At sufficiently high temperatures, carbon dioxide is removed from the carbonate only by the addition of other chemicals. In principle, the following three reaction steps can be shown for an alkali, where an inorganic acid is
Also, the alkali-consuming organic matter during cooking, such as lignin, is expressed as LignOH. 1 Cooking or bleaching (delignification) Nam+, Hn-, A+LignOH2LignONa
+NamHnA2 combustion public ignONa+x. 02→Na
2C0310y. C02 10z. Ratio 03 Automatic causticizing 2Na
mHnA+Na2C○3 Koga am+, Hn[, A+C020 QO The principle of autocausticization is that when one or several reaction products (in this example C02 and Day20) are removed from the system, an acid weaker than carbon dioxide is used. It is based on the fact that carbon dioxide can be driven out of carbonates using (HnAm-).

In other words, the equilibrium in the third reaction step can be shifted to the left, but since C02 and 20 are continuously released, the equilibrium can be shifted to the right and the products Nam+, Hn-, and A can be obtained in theoretical yields. . However, the cooking chemicals are in non-causticized and causticized states (Nam is A and Nam+, H, A
) are non-volatile, and this also applies to the common sodium salts. Causticizing product Nam+, ratio-
, A is sufficiently alkaline, the product is useful as a delignification chemical.

This is the case, for example, when using secondary sodium borate Na2H803, which is shown to correspond approximately equimolarly to NaOH, as the effective alkali during the alkaline pulping process. In this case, the autocausticizing reaction is as follows. 2
Na & B03 ten N C08 → 2Na2HB03 ten C02
The salt Na2HB03 containing 10QO does not work in its dry state, but is dehydrated according to the following reaction formula.

2Na2HB03Na4B20520 However, by redissolving the above salt in water, the salt is hydrolyzed inversely to the orthoborate.

Next, Na2HB03, common Na, Hn-, and A represent the corresponding polynuclear ion salts. The conditions required for autocausticization depend on the nature of the chemical in question.

As shown in FIG.
: B equals 2.0) at 875 qo for 3 hours to achieve a degree of causticization of 80% (initial CO
It turns out that 20% of the amount remaining) can be achieved. A causticizing experiment using the same borate-carbonate mixture as above was carried out at 72
o~3 steps were taken within the temperature range of 5 to 87,500°C (see Figure 1). It is clear that carbonic acid decomposition follows approximately first-order kinetics, and the reaction constant k (sec-1) can be calculated using the following formula: .

The activation energy is 111 KJ/mol, which means doubling the reaction rate when increasing the temperature from, for example, 875 CO to 9470 CO. If the molar ratio F changes,
It can be seen that causticization is easy with Fmi2 and not so easy with F>2 (see Figure 2). This also means that from F>2 the mixture is Na
It should be expected that the carbon dioxide would consist of H2B03 and Na2C03, and that carbon dioxide would not be a strong enough acid to
One can expect removal by ion day 2B03- (forming HB032-) than by ion day 2-2-. Similar experiments with phosphates show that the following automatic causticization can be performed.

2Na2f press 04 ten Na2C03 →2Na3P04
10C02 10th 20As shown in Figure 3, 525oo is about 4
After 0 minutes 80% of carbon dioxide was removed, 625-725
00 removes more than 90%.

When the molar ratio G=Na:P is 3 or more, causticization is incomplete (see FIG. 2), and no causticization occurs in GZ4. For example, if G=3.5 the mixture is half causticized (
Figure 3). Na3PQ+Na2HP.

40 Na2C.

3 states a3P.

40 Ura Na2CQ+.

20 history.

That is, for borate- and phosphate-containing waste liquids,
It is essential to maintain F=Na/BS2 and G=Na/PMi3 respectively to ensure complete causticization.

Other ampholyte salts can be used as well, such as silicates and aluminates. Experiments were conducted using organic materials present during heating of borates and phosphates in the presence of air to simulate actual combustion and causticization of waste fluids.

That is, Na2HB03 and Na3P04 are mixed with vanillin and glucose (and some water), respectively,
Heated in a laboratory furnace. Causticization proceeds more slowly than when pure carbonates are present instead of organic compounds (first
(see figure). An experimental example using an actual pulping waste liquid (a birch waste liquid corresponding to a pulp yield of 65 to 79%) is shown below.

Causticity refers to the fraction of carbonates that are formed during combustion and remove CO2 during heating.

It can be seen from the table that the composition of the raw cooking liquor and the main products after dissolution are the same. That is, borate and phosphate waste liquors can be burned and regenerated by heating (autocausticizing) in such a way as to form a liquid that can be reused as an alkali for pulp production.

Kraft and "soda" cooking in which bleaching, such as oxygen bleaching, can be carried out can be carried out using borates or phosphates instead of hydroxides as the alkali IJ.

An example of birch kraft cooking at a liquid to wood ratio of 3.6 and an H factor of 1 is shown below.

An example of "soda" cooking of birch wood (35 ratio of liquid to wood of 4.0) is shown below.

During the kraft and alkaline cooking of birch wood, 1 mol of Na2HB03 becomes 1.2 mol of NaOH,
Also, 1 mol of Na3P04 is about 0.5 to 0.6 mol of N
It turns out that it corresponds to aOH. The following oxygen bleaching experiment can be given as an example of the use of slightly alkaline NaH2803.

The starting material is birch alkaline bulb with an IJ gunin content of 21.7% that was cooked to a yield of 67.4%. During bleaching, the pulp consistency was 10%, the oxygen pressure was 8 bar, the maximum temperature was 120 qo, and the time was 45 minutes at 120 qo. In this case, the advantage of using a weak alkali is that the yield can be increased by about 2% or more at a given lignin content.

In the present invention, Na is used instead of hydroxide during pulp treatment.
An alkaline borate such as 2HB03 can be used or the organic matter in the waste liquor can be burned to carbonate and the residue can be causticized by heating to obtain a fresh alkaline liquor suitable for use in the pulping process. can.

Alkali loss during the pulping cycle can be compensated by hard sand and soda. Similarly, bleaching alkalis can be produced and other inorganic chemicals can be used.

[Brief explanation of the drawing]

Figure 1 is a curve diagram showing the causticization degree by causticization treatment using a borate-carbonate mixture, and Figure 2 is a curve diagram showing the relationship between the Na:B or P molar ratio and the causticization degree in automatic causticization. FIG. 3 is a curve diagram showing the degree of causticization at a given Na:P molar ratio in autocausticization. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. In alkaline pulping of wood cellulose material or alkaline bleaching of pulp derived from wood cellulose material, NaH_2BO_3 and Na_2HBO_3
Cooking the wood cellulosic material or bleaching the pulp in the presence of oxygen in an alkaline aqueous cooking liquor or bleaching liquor containing as active ingredient an alkaline salt of at least one polybasic inorganic acid selected from the group consisting of alkaline inorganic acids; A process for alkaline pulping or alkaline bleaching, characterized in that the remaining cooking liquor is combusted so as to obtain a substance, and the said inorganic substances are dissolved in water to give the same alkali salts used in the cooking or bleaching process. 2. The method according to claim 1, wherein the liquid is evaporated before combustion. 3. The polybasic inorganic acid is boric acid, the borate in the remaining cooking liquor has a Na:B ratio of 1 to 2 (including Na present as Na_2S), and at a temperature of 200 to 1500 °C A method according to claim 1 in which combustion is performed.