JPS6364559B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to alkaline pulping processes, particularly the soda and sulfate processes when the feedstock is contaminated with soluble silica.

Alkaline pulping methods using wood and non-wood raw materials are known and produce cellulosic fibers in a cooking liquor containing caustic soda alone or with a predetermined amount of sodium sulfide as the main delignification component. It consists of cooking and treating raw materials. This cooking treatment is carried out under high temperature and high pressure conditions. In general, the soda method using sodium hydroxide alone is suitable for soft non-wood materials and other cellulosic fiber materials, while the sulfate method using sodium hydroxide and sodium sulfide is suitable for pine wood, oak wood, eucalyptus wood, etc. This method is suitable for wood such as wood and dense non-wood materials such as bamboo.

Briefly, in the alkaline method, cellulosic fiber raw materials are steamed in an alkaline cooking liquid under high temperature and high pressure, the black liquor obtained by steaming is separated from the pulp in a brown stock washer, and the washed pulp is further processed. , and route the black liquor to a recovery system.
Here, it is necessary to first concentrate the black liquor in multiple evaporation cans, and then send the concentrated black liquor to a furnace that burns off the organic matter and leaves behind sodium salts, including sodium carbonate. The sodium salts from this furnace are dissolved in a wash liquor containing dilute cooking liquor to make what is called green liquor. This green liquor is reacted with lime in a causticizing step to form sodium hydroxide, which can be recycled into the system, and calcium carbonate, which when calcined becomes lime, which can be recycled into the system.

The raw materials used in the alkaline process, such as bamboo, bagasse (sugar cane husks), wheat and rice straw, and tree bark, carry the pollutant soluble silica into the system. Once soluble silica reaches the digester, it tends to dissolve and thicken the black liquor. In multiple evaporation cans, quartz scale adheres to the inner surface, so
The efficiency of the evaporator can is reduced and the causticizing process forms deposits of calcium silicate which are recycled to the lime recovery system. This entrainment of calcium silicate into the recovery system must ultimately be prevented by removing this inert component from the system. Since calcium silicate does not turn into lime when calcined, most of the contaminated lime must be removed from the system. Therefore, it is necessary to replenish this amount of lime.

The silica that is a problem in papermaking is soluble silica, not insoluble silica that exists in the form of sand or quartz. Soluble silica is absorbed during plant growth, particularly by plants, and can sometimes be present in large concentrations locally in plants, such as in bamboo nodes.

Literature (Chem.Abstracts.1968, Vol.68, 8536
Page) discloses the use of CO ₂ , MgSO ₄ or Al ₂ (SO ₄ ) ₃ to desilica bamboo kraft pulp black liquor. According to this literature, Al ₂ (SO ₄ ) ₃ is superior to MgSO ₄ and CO ₂ in terms of removing silica, but when considering economic efficiency,
Al ₂ (SO ₄ ) ₃ must be recovered and reused. Of course, the high cost of Al ₂ (SO ₄ ) ₃ precludes desilication using it, and recovery of this compound for reuse requires high capital and energy costs. processing is required.

Thus, it is clear that there is a need for a method that can remove soluble silica before it adversely affects the pulp manufacturing process.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method for removing silica at an early stage of the process in an alkaline pulp manufacturing process.

It is also an object of the present invention to provide an improved alkaline pulp production method that allows the use of silica-contaminated feedstock in producing pulp.

Another object of the present invention is to provide a relatively simple and therefore economical method for removing soluble silica in an alkaline pulping process.

Other objects and advantages of the present invention will become apparent from the following detailed description in conjunction with the accompanying drawings.

The following discussion employs some terms used in the pulp/paper industry, with definitions as follows.

Black liquor: organic matter and residual caustic Na ₂ CO ₃ and
Green liquor consisting of NaHS: Burnt product of black liquor (melt or pellets)
A liquid in which Na ₂ CO ₃ is dissolved White liquor, i.e., cooking liquid: The following formula Na ₂ CO ₃ + CaO + H ₂ O → 2NaOH
+ CaCO ₃ ↓ A NaOH-containing liquid pulp liquor obtained by reacting green liquor and lime according to: white (cooking) liquor or waste black liquor Generally speaking, the present invention combines silica and sodium hydroxide present in the system. enough to react,
Improvement of an alkaline pulp production method for producing pulp from silica-contaminated raw materials by introducing a predetermined amount of aluminum oxide, especially bauxite ore (consisting of hydrated aluminum oxide and impurities) into the pulp liquor to precipitate silica as sodalite. Regarding. Hydrated aluminum oxide is sensitive to the presence of NaOH,
It consists of monohydrate Al ₂ O ₃ .H ₂ O (diaspor) or trihydrate Al ₂ O ₃ .3H ₂ O (gibbstone). The latter gibbstone is stoichiometrically 2Al
(OH) equals ₃ . For simplicity of explanation, the term "aluminum oxide" is adopted herein to refer to hydrated aluminum oxide compounds.

It is also possible to add bauxite (hydrated aluminum oxide) to the white liquor to obtain an intermediate oxide compound which is soluble in NaOH and which later precipitates silica as sodalite.

More specifically, bauxite can be added at several points in the pulping process. That is,
(a) Add bauxite directly to the black liquor before or during evaporation. (b) Dissolve the bauxite in white liquor and then add this white liquor to the digester along with the sodium aluminate produced. Incidentally, the precipitation of sodalite starts from the time when bauxite is introduced.

When carrying out the present invention, aluminum oxide contained in bauxite ore is converted into trihydrate (Al ₂ O ₃
3H ₂ O). This is because, although monohydrate (Al ₂ O ₃ .H ₂ O) also performs the desired function, it is generally not readily soluble.

Bauxite ore is a cheap source of aluminum oxide. Because bauxite is naturally occurring, it can exist in the form of a mixture of monohydrate and trihydrate, and in some people Al ₂ O ₃ .
Some people write it as 2H ₂ O. Considering this, bauxite containing Al ₂ O ₃ .2H ₂ O also falls within the scope of the present invention.

As mentioned above, soluble silica is removed by adding bauxite (aluminum oxide) to precipitate the silica as sodalite. This reaction can be explained by the following typical reaction formula.

3SiO ₂ +4NaOH+3Al(OH) ₃ →
Na ₄ Al ₃ (SiO ₄ ) ₃ OH・2.4H ₂ O↓+3.6H ₂ O The exact structure of sodalite depends on the chemistry of the raw materials used and other factors. For example, OH
can be partially replaced by 1/2CO ₃ , thus
To be precise, the degree of hydration does not correspond to 2.4H ₂ O.

Next, referring to FIG. 1 which shows a flow diagram of the well-known soda process or sulfate process, although some new processing means have been added, a predetermined amount of chips or other cellulose-based The fibers are fed into the digester 11 and pipes 7 and 9 respectively.
The black liquor and white liquor are sent to the digester by a pipe 5, while steam is also sent to the digester by a pipe 5 to heat and pressurize the digester. Although the cooking liquor in the digester dissolves most of the lignin from the chips, it has only a relatively small effect on cellulose.
The cellulose remains in a form suitable for conversion into subsequent paper. The active ingredients of the cooking liquor in the sulfate process are sodium hydroxide and sodium sulfide. Preferably, the digester operates at a temperature of 165-177°C (330-350°) and a pressure of 100-150 psi.
The black liquor and pulp products in the digester 11, still under pressure, are transferred by pipe 13 to a blow tank 14.
to reduce the pressure to near ambient pressure. In order to recover and recycle used chemicals, the liquid from the blow tank must be separated from the pulp. A brown-stock washer, into which pulp liquor and black liquor are already fed via pipe 15,
In the washer 18, this separation takes place by sending the washed pulp removed via a tube 20 to, for example, a screen room and/or a bleaching plant or a paper mill for further processing. Water is introduced into the brown stock washer 18 by conduit 17 for cleaning, and dilute black liquor is removed by conduit 19 and sent to a recovery system. The steps used in the recovery system are as follows.

(a) Concentration of black liquor (b) Combustion of concentrated black liquor (c) Dissolution of the solid combustion material in a mild washing liquor (d) Production of a cooking liquor by causticization with lime of the melt (e) Calcination of lime mud for reuse in causticizing.

In the multiple evaporator cans 22 sufficient water is removed from the black liquor stream introduced via tube 19 to enable it to be combusted efficiently. The concentrated black liquor, which is the product of the multiple evaporation cans and which is sent to the recovery furnace 26 by pipe 23, contains organic matter and sodium compounds in solution. This concentrated black liquor is spontaneously combusted in the recovery furnace. After combustion in the recovery furnace 26, residues of sodium carbonate, unburned carbon, and non-combustible minerals contained in the chips or other cellulosic fiber materials remain. The recovery reactor product, which is "black ash", either melt or pellets, is conveyed by pipe 27 to melting tank 30. In the dissolution tank, the recovery furnace product in the form of melt etc. is dissolved in the weak washing liquid introduced by pipe 33 from the next step, lime mud washing, to produce a solution consisting mainly of sodium carbonate. do. In the causticizing step, sodium carbonate from the melt treated with lime is converted to caustic soda (sodium hydroxide), and the reaction is as follows.

CaO＋H ₂ O→Ca(OH) ₂
Ca(OH) ₂ +Na ₂ CO ₃ →CaCO ₃ ↓+2NaOH In this way, in the causticizing system 34, the tube 31
By processing the green liquor sent here by
NaOH is produced which is then returned to the digester via line 9 as white liquor. The lime mud from the causticizing system 34 is conveyed by pipe 35 to a calciner 38 for lime reburning. In the calciner, calcium carbonate is heated to high temperatures and carbon dioxide is released.
Reburned lime is returned to the causticizing system by pipe 37.

Systems of the type described above are in use throughout the world, and the results have been satisfactory.
However, problems arise when the raw material is contaminated with soluble silica. Such contamination is mainly caused by the use of bagasse (sugar cane husks),
This occurs when wheat straw, rice straw, esparto, bamboo, etc. are used.

Until now, this silica was dissolved early in the process and then sent to the evaporation stage along with the black liquor.
It was sent to the causticizing system along with the melt and green liquor. However, as previously mentioned, silica thickens the black liquor, resulting in scale formation in the evaporator can and increased turnaround time in the causticizing system.

According to the present invention, when bauxite (aluminum oxide) is introduced into the pulp liquid, silica is precipitated as sodalite. The precipitated sodalite can be removed from the black liquor by settling and filtration. Therefore,
In the method of FIG. 1, a silica-containing liquid is brought into contact with bauxite. Alternatively, bauxite (aluminum oxide) may be added to the white liquor flowing from the causticizing system to digester 11 through tube 9' instead of 9.
Add. A reaction/dissolution stage 91 is provided in which bauxite supplied by tube 90 is transferred to tube 9'.
After mixing with the white liquor supplied by, the mixture is sent to a separation stage 92 to remove impurities such as Fe ₂ O ₃ .

Figure 2 shows the introduction of bauxite at different times, in this case reactor 181 and separator 1.
82, brown stock washer 18 and evaporator can 22
Provided between. The dilute black liquor from brown stock washer 18 is sent to reactor 181 by pipe 19. Meanwhile, bauxite is sent to reactor 181 via pipe 170. In reactor 181, the aluminum oxide of bauxite reacts with dilute black liquor and silica is precipitated as sodalite. Reactor 181
The contents of the system are conveyed by line 191 to separator 182 where the black liquor is separated from the precipitated sodalite, which is removed from the system stream by line 193. On the other hand, the black liquor is sent to the next stage, an evaporation can 22, and then sent to a recovery furnace 26 via a pipe 23.

FIG. 3 shows the introduction of bauxite at yet another point, midway through evaporation, in which dilute black liquor from brown stock washer 18 is introduced into first stage evaporator can 220. At this early stage of evaporation, scale formation and thickening are not much of a problem. The partially thickened black liquor is sent via line 195 to reactor 221 where it is reacted with the aluminum oxide of the bauxite introduced into reactor 221 via line 80. The reaction products are transferred from the reactor via tube 196 to separator 222 where the precipitated sodalite is separated from the black liquor and transferred to tube 1.
98 from the system stream. The remaining black liquor is sent through a pipe 197 to a subsequent evaporation stage 223, where it is evaporated, and then sent through a pipe 23 to a recovery furnace 26.

Although not shown in FIGS. 2 or 3, it is also possible to provide a final separation stage immediately before the recovery furnace 26 in order to remove sodalite that is deposited late during the concentration of the black liquor.

Because the price of bauxite is relatively low, it may be better to discard the precipitated soda stone rather than processing it for chemical recovery. Tube 9' in which Al ₂ O ₃ is leached from the bauxite before being added to the digester 11
The addition of bauxite through this method was specially devised in consideration of cases where bauxite contains harmful insoluble substances such as fine Fe ₂ O ₃ that cannot be removed by the bleaching process. Therefore, after the reaction/dissolution stage 91,
A separator 92 is provided for separating and removing Fe ₂ O ₃ and/or other insolubles from a tube 93, which are connected by a tube 9'. A clear white liquor containing Al ₂ O ₃ in the form of Na ₂ O.Al ₂ O ₃ flows from separator 92 to digester 11 . In this case, it may not be necessary to remove sodalite from the pulp. This is because impurities that cause discoloration and other impurities have already been removed in separator 92.

Examples are now given to illustrate the advantages of the invention.

Examples - Methods Demyelinated bagasse was obtained from a bagasse pulp mill. The moisture content of this bagasse was 76.0%, and the ash content when burned at 550°C was 4.02% of the oven-dried (OD) bagasse. 75.1% of the ash content was _SiO2 .

Tests were carried out in 4 autoclaves in which 180 g of bagasse (OD basis) was steamed in soda white liquor.

Total steaming yielded 71.3 gpl of NaOH and 8.6 gpl of
750 g of wet bagasse was treated with 306 ml of white liquor containing Na ₂ CO ₃ . Added 500ml of water to the digester.
The steaming cycle consisted of 4 minutes to the specified temperature (171°C) and pressure (110 psig), 10 minutes of steaming, and
It took 6 minutes to cool down to 100°C. After this,
After filtering and washing the cooked pulp, the black liquor and pulp were analyzed.

Control Sample In this case, the silica-contaminated bagasse was steamed as described above and the amount of silica present in the dissolved solids of the black liquor was determined, and the black liquor contained a total of 6.85% dissolved solids; Of this, silica accounted for 0.1707%. That is, silica was 2.5% of the dissolved solids. 1350 g of black liquor obtained by steaming bagasse contained 2.3 g of soluble silica. The presence of this amount of silica in the black liquor caused significant scale formation problems in the evaporation process, unacceptably thickened the black liquor, and resulted in significant lime loss in the causticizing process.

Example Bauxite in white liquor (see tube 90 in Figure 1)
In a mock test of the method, white liquor was cleaned to remove residual bauxite and then pumped into a digester to react with soluble silica.
Dissolve the hydrated sodium aluminate in this white liquor. The white liquor contained 1.95 g Al ₂ O ₃ stoichiometrically equivalent to 2.29 g SiO ₂ . Applying the steaming cycle and washing described previously, the total dissolved solids content of the black liquor was 7.06%, of which SiO ₂ and
Al ₂ O ₃ min were 0.011% and 0.063%, respectively.
Therefore, it can be seen that this treatment is effective as the silica content of the total dissolved solids decreased from 2.5% in the control sample to 0.16%.

A slight reduction in the amount of Al ₂ O ₃ is also effective.
For example, when applying a stoichiometric ratio of 0.043% Al ₂ O ₃ , SiO ₂ is 0.0286%.

EXAMPLE Immediately before adding the white liquor to the bagasse in the digester, 5.3 g of -65 mesh bauxite is added to the white liquor. The composition of bauxite is the same as in the previous test. This bauxite contains 4.25 g of Al ₂ O ₃ (ignoring the SiO ₂ of the bauxite), which is more than 85% of the amount needed to react with the silica. The black liquor containing a total of 6.6% dissolved solids contains 0.0277% _SiO2 and 0.0301%
It contained Al ₂ O ₃ . From this, it can be seen that the amount of bauxite is slightly excessive. _{Al2O3 _}
In terms of stoichiometric ratio to _SiO2 ,
_Al2O3 and _SiO2 are _0.0266 % and 0.0312% respectively
Must.

EXAMPLE 5.6 g of -65 mesh bauxite is added to the white liquor in the digester. The black liquor with a total dissolved solids content of 6.25% has only 0.0161% _SiO2 ;
Al ₂ O ₃ min is at most 0.0606%.

The above examples clearly draw the conclusion that silicon-contaminated pulp liquors can be economically desilicated simply by adding hydrated aluminum oxide, especially aluminum oxide in the form of natural aluminum ore or bauxite.

Although the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced in various ways without departing from the spirit and scope of the invention. Therefore, embodiments other than those described should be included within the scope of the invention.

[Brief explanation of drawings]

FIG. 1 is a flow diagram showing an alkaline pulp manufacturing method to which the present invention can be applied;
3 is a partially modified flow diagram of FIG. 1 showing another embodiment of the invention, and FIG. 3 is a flow diagram of FIG. 1 showing a further embodiment of the invention. This is a partially changed diagram. 11: Digester, 14: Blow tank, 18: Washer, 22: Evaporator can, 26: Recovery furnace, 30: Melting tank, 34: Causticizing system, 91: Reaction/dissolution stage, 92: Separation stage, 181: Reactor, 182:
Separator, 220: first evaporation can, 222: separator.

Claims (1)

[Claims] 1. Cook the cellulosic fiber raw material with white liquor containing sodium hydroxide, separate the produced black liquor from the pulp, concentrate the black liquor in multiple evaporation cans, and burn the concentrated black liquor to produce black Silica produced in an alkaline pulping process comprising the steps of producing an ash, melt or pellets and treating the ash, melt or pellets to recover sodium hydroxide in the form of a white liquor which can be recycled into the cooking process. In the method of desilicating contaminated pulp liquor, bauxite ore is introduced into the white liquor to form an intermediate compound, sodium aluminate, which reacts with the silica, and the solid impurities introduced together with the bauxite ore are removed and this impurity is removed. A desilicament process characterized by removing the white liquor/sodium aluminate solution from the system and sending it to a cooking process. 2. The desilication method according to claim 1, wherein the solid impurity separated and removed in the separation and removal step is Fe ₂ O ₃ . 3. The desilication method according to claim 1 or 2, wherein the aluminum oxide contained in the bauxite ore is in the form of trihydrate. 4 Cooking cellulosic fiber raw materials with white liquor containing sodium hydroxide, separating the produced black liquor from the pulp, concentrating the black liquor in multiple evaporation cans, and burning the concentrated black liquor to produce black ash, melt, or pellets. The silica-contaminated pulp liquor produced in the alkaline pulping process consists of the steps of producing ash, melt or pellets to recover sodium hydroxide in the form of a white liquor that can be recycled into the cooking process. In the method, bauxite ore is introduced into the black liquor to form an intermediate product, sodium aluminate, which can react with the silica in the black liquor to precipitate sodalite, and the precipitated sodaite is separated and , a method for desilication, characterized in that this precipitate is removed from the flow of the system. 5. The desilication method according to claim 4, wherein bauxite ore is added to the black liquor prior to the concentration step. 6. The desilification method according to claim 4, wherein aluminum oxide is added to the black liquor at an intermediate stage of evaporation. 7. Process according to any one of claims 4 to 6, in which aluminum oxide is added in the form of bauxite ore in trihydrate form.