GB2065188A - Desilication in Alkaline Pulp Processes - Google Patents

Abstract

In the alkaline digestion of silica- contaminated paper pulp raw material, an addition of a hydrated aluminum oxide, e.g. bauxite, is made to the liquor, to insolubilize silica as a sodalite precipitate, and thereby minimize scaling during evaporation, avoid undesirable increase in the viscosity of the liquor, as well as assuring recovery of lime by avoiding formation of calcium silicate during causticizing.

Description

SPECIFICATION Desilication in Alkaline Pulp Processes This invention relates to alkaline-type pulping processes for the manufacture of pulp; that is, the soda and sulfate processes, particularly where the raw materials used are contaminated with soluble silica.

The alkaline-type wood and non-wood pulping processes are well known and involve the digestive treatment of the cellulosic fiber material in cooking liquors which contain caustic soda alone or with substantial quantities of sodium sulfide as the principal delignifying constituents of the liquors, the digestive treatment usually being carried out under elevated temperature and pressure conditions.

In general, the soda process, employing sodium hydroxide alone, is adequate for the softer non-woods and other cellulosic fibrous materials, and the sulfate process, in which both sodium hydroxide and sodium sulfide are employed, is suitable for woods such as pine, oak or eucalyptus and for dense nonwoods such as bamboo.

In broad outline the alkaline processes call for cooking the cellulosic fiber material in an alkaline cooking liquor at elevated temperature and pressure, separating the black liquor generated by the cooking process from the pulp in brownstock washers, forwarding the washed pulp for further processing, and routing the black liquor into a recovery processing system in which the black liquor is first concentrated in multi-effect evaporators and the concentrated liquor directed into a furnace in which the organic material is burned, leaving sodium salts including sodium carbonate.The sodium salts from the furnace are dissolved in wash liquors containing dilute cooking liquor, forming the socalled green liquor, the green liquor is reacted with lime in the causticizing step, producing sodium hydroxide which can be recycled in the process and calcium carbonate which is calcined to produce lime for recycling in the process.

Some raw materials used in the alkaline processes such as bamboo, bagasse (waste sugar cane stalks from which the juice has been extracted), wheat and rice straw and wood bark, carry contaminating soluble silica into the process. Soluble silica which reaches the digester tends to dissolve in the black liquor and increases the black liquor viscosity. In the multi-effect evaporators, siliceous scale deposits on the internal surfaces thereby reducing the effectiveness of the units, and in the causticizing step, calcium silicate is formed, retained and recirculated in the lime recovery process.

This burden of calcium silicate in the recovery process must ultimately be relieved by withdrawing this inert constituent from the process. The calcium silicate cannot be calcined to form lime and therefore a substantial amount of the contaminated lime must be removed from the cycle, and lime required for the process must be purchased fresh lime.

The silica which causes the difficulty in the paper pulp processes is the soluble silica, not silica in the insoluble form; i.e., sand or quartz. The soluble silica is the silica absorbed by the plant in its growth and is sometimes found in localized concentration in the plant, for example, in the nodes of the bamboo plant.

It has been reported in the literature that CO2, MgSO4 or Al2 (SO4)3 have been used to effect desilication of bamboo kraft pulp black liquor (Chem. Abstracts, 1968, Vol. 68, Page 8536). Al2 (S04)3 is regarded in the reported work as superior to MgSO4 or CO2 in the desilication application, but the recovery and reuse of the Al2 (S04)3 is recognized as essential from the viewpoint of economics. As a matter of fact, Al2 (S04)3 is initially so expensive as to render desilication by this means impractical and, further, the essential recovery of the compound for reuse involves processiny which is both capital and energy cost intensive.

It is clear that a need exists for a process which is capable of removing soluble silica before its deleterious effects on the process occur.

It is accordingly an object of the invention to provide a method in the alkaline-type pulping processes for removing silica at an early stage in the processing.

It is an object of the invention to provide an improved alkaline pulping process in the manufacture of pulp which can utilize silica-contaminated raw materials.

It is still another object of the invention to provide a relatively simple and economical method for removing soluble silica in an alkaline-type pulping process for pulp manufacture.

Other objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a flow diagram showing an alkaline-type pulping process to which the invention may be applied, Fig. 2 is a modification of a portion of the flow diagram of Fig. 1 showing another embodiment of the invention, Fig. 3 is a modified form of a portion of the flow diagram of Fig. 1 showing still another embodiment of the invention, Fig. 4 is a schematic showing of one embodiment of the invention.

The following definitions of terms used in the pulp and paper industries will be employed in the description set forth hereafter: Biack Liquor--iiquor comprising organics resulting from the digestion of raw cellulosic plant material together with residual caustic, Na2CO3 and NaHS.

Green Liquor-liquor comprising dissolved Na2CO3; the Na2CO3 being a product (smelt or pellets) of the combustion of black liquor.

White or Cooking Liquor-Liquor containing NaOH obtained by reacting the green liquor with lime (CaO) in accordance with the following equation: Na2CO3+CaO+ H20 > 2NaOH +CaC03$ Pulp Liquor-white (cooking) liquor or spent black liquors Generally speaking, the present invention is directed to an improvement in alkaline-type pulping processes for the manufacture of pulp from silica-contaminated raw materials wherein a quantity of aluminum oxide, selected from the group consisting of the hydrated aluminum oxides, sufficient to react with the silica and sodium hydroxide present in the process is introduced into the pulp liquor to precipitate the silica as a sodalite. To be readily soluble in the NaOH present, the aluminum oxide must, in general, be hydrated.The hydrated oxide could be precipitated aluminum hydroxide, Al(OH)3, the monohydrate, Awl203 H2O (diaspore), or the trihydrate, Awl203 3H20 (gibbsite), the latter, of course, being stoichiometrically equivalent to 2AI(OH)3. For the sake of clarity, the term "aluminum oxide" is used herein to cover all of the above hydrated aluminum oxide compounds.

The hydrated aluminum oxide may be added to the white liquor to produce the intermediate oxide compound Na20 awl203, which is soluble in NaOH and will precipitate the silica as a sodalite.

More specifically, it has been found that there are several points in the pulping process at which the soluble aluminum oxide addition may be made, as follows: a) aluminum oxide may be added directly to the black liquor either prior to the evaporators or at an intermediate stage of evaporation.

b) aluminum oxide may be dissolved in the white liquor; the white liquor, with the sodium aluminate thus formed, then being added to the digester.

c) The aluminum oxide may be introduced directly into the digester.

It will be understood that sodalite precipitation ordinarily commences at the point of introduction of the aluminum oxide.

In carrying out the invention, it is preferred that the aluminum oxide be in the form of the trihydrate (Al203 3H20), for the reason that, although the monohydrate (awl203 H20) will perform the desired function, the latter, in general, is less readily soluble.

It has been found, moreover, that bauxite ore may be used as a source of the aluminum oxide (again, the trihydrate form is preferred) where the availability of bauxite provides an inexpensive source for the aluminum oxide. As it occurs in nature, bauxite is sometimes a mixture of the tri and mono hydrate and has been designated as Al2O3 2H20 by some authors. In this light, Al203 2H20 would aiso come within the scope of the invention.

As indicated previously, the removal of soluble silica is effected by precipitation of the silica as a sodalite by the addition of aluminum oxide. The following exemplary equation illustrates this reaction: 3SiO2+4NaOH+3Al(OH)3oNa4AI3(SiO4)30H$2.4H20;+3.6H20 The exact formula of the sodalite may vary depending upon the chemistry of the raw materials used in the process and other factors. For example, the OH may be replaced in part by 1/2CO3 and the degree of hydration may not exactly conform to 2.4H20.

Turning to Fig. 1, which depicts a conventional soda or sulfate process flow diagram with certain additions, it is seen that the digester 11 is supplied with a feed of wood chips or other cellulosic fiber through line 3, black liquor is supplied through line 7 and white liquor is supplied to the digester through line 9, while steam to heat and pressurize the digester is supplied through line 5. The cooking liquor in digester 11 dissolves most of the lignin from the wood, with relatively slight effect on cellulose, thus leaving the latter in a form suitable for subsequent conversion into paper. The active constituents of the cooking liquor in the sulfate process are sodium hydroxide and sodium sulfide.The digester is preferably operated at a temperature in the range from 1 650C to 1 770C (roughly 330 to 3500F) and preferably at a pressure in the range from 100 to 1 50 psi. The black liquor and pulp product of the digester 11, still under pressure, is discharged into blow tank 14 through line 1 3 thereby reducing the pressure to essentially ambient pressure. The liquor and pulp from the blow tank must be separated to remove spent chemicals for subsequent recovery. In the brown-stock washers 18, to which the pulp and black liquor have been conveyed through line 15, this separation is carried out with the washed pulp removed through line 20 for further processing, for example, to a screen room, and/or bleach plant or paper mill. Water is introduced by means of conduit 17 into the brownstock washers 18 to carry out the washing procedure and a dilute black liquor stream is removed through line 19 to the recovery system of the process. In the recovery system the stages employed are: a) concentration of black liquor; b) combustion of concentrated black liquor; c) dissolving the combustion solid products in weak wash liquors; d) causticizing the dissolved product with lime to produce cooking liquor; e) calcining the lime mud for reuse in causticizing.

In the multi-effect evaporators 22 enough water is removed from the black liquor stream introduced through line 1 9 to enable it to be burned efficiently. The concentrated black liquor, which is the product of the multi-effect evaporators 22 and which is forwarded to the recovery furnace 26 through line 23, contains organic matter in solution and sodium compounds. The concentrated liquor will burn autogenously in the furnace. After the combustion stage, which is carrried out in the recovery furnace 26, there remains a residue of sodium carbonate, unburned carbon and incombustible inorganic matter from the wood or other cellulosic fibrous material. This product of the recovery furnace, which may be a "black ash", smelt of pellets, is forwarded to the dissolving tank 30 through line 27.In the dissolving tank, the smelt or other product of the recovery furnace is dissolved in weak wash liquor introduced through line 33 from lime mud washing, a subsequent stage in the process, thereby providing a solution primarily comprising sodium carbonate. The causticizing process consists in converting the sodium carbonate from the dissolved smelt into caustic soda (sodium hydroxide) by treatment with line, the reactions being as follows: Ca0+H2OCa(0H)2 CafOH)2TNa2CO3eCaCOs;+2 NaOH Thus, in the causticizing system 34 the green liquor directed thereto through line 31 is treated to produce NaOH, which is then returned to the digester as white liquor through line 9. The lime mud from the causticizing system 34 is directed as indicated by line 35 to a calciner 38 for lime reburning.In the calciner, the calcium carbonate is heated to an elevated temperature, carbon dioxide is driven off and the reburned lime (CaO) is recycled through line 39 to the causticizing system.

A system of the type just described has been successfully installed in many different locations around the world. A problem does arise, however, when the raw materials are contaminated with soluble silica. Such contamination is observed principally in locations where materials such as bagasse, wheat and rice straw, esparto grass and bamboo are employed in pulp manufacture due to the shortage or absence of pulping grade wood.

Once in the process, the silica is dissolved in the initial stages of treatment and is transported with the black liquor, into the evaporation stage and with the smelt and green liquor into the causticizing system. As indicated previously, the silica increases the viscosity of the black liquor, results in scale formation in the evaporators and in a loss of lime in the causticizing system.

In accordance with this invention, the introduction of aluminum oxide into the pulp liquor will precipitate the silica as a sodalite. The sodalite precipitate can be removed from the black liquor by settling and filtration. Thus, in the Fig. 1 process, the objective is to contact the silica-bearing liquor with the aluminum oxide. One straight-forward example of effecting this contact is indicated by the line 100 through which the aluminum oxide may be introduced directly into the digester 11 to contact the cooking liquor therein.An alternative is to add the aluminum oxide tothe white liquor flowing in line 9' (substituted for line 9) from the causticizing system to the digester 11. A reaction-dissolving stage 91 is provided in which the aluminum oxide supplied through line 90 is mixed with the white liquor supplied through line 9'.

In Fig. 2 another alternative for introduction of aluminum oxide into the process is illustrated. In this case, a reactor 181 and a separator 182 are inserted between the brownstock washers 18 and the evaporators 22. Dilute black liquor from the brownstock washers 18 is directed into reactor 181 through line 19. Aluminum oxide is routed into reactor 181 through line 70. In reactor 181 the aluminum oxide reacts with the dilute black liquor and the silica is precipitated as a sodalite.The contents of reactor 1 81 are directed into separator 1 82 through line 1 91 and the black liquor is separated therein from the sodalite precipitate; the separated sodalite being removed from the process stream through line 1 93 and the black liquor being advanced to the next stage of the process, the evaporators 22 and, thereafter, to the recovery furnace 26 through line 23.

In Fig. 3, still another point in the process at which the aluminum oxide may be introduced, an intermediate stage of evaporation, in indicated. In this case, dilute black liquor from brownstock washers 1 8 is introduced into the first stage evaporator 220. Neither scaling nor viscosity increase are especially troublesome in this early stage of evaporation. The partially concentrated black liquor is directed by means of line 1 95 into reactor 221 where reaction occurs with aluminum oxide introduced into reactor 221 through line 80.From the reactor, the reaction products are conveyed by line 196 to separator 222, in which the sodalite precipitate is separated from the black liquor and removed from the process stream through line 1 98. The black liquor is forwarded through line 197 to additional stages of evaporation 223 and, following evaporation, to the recovery furnace 26 through line 23.

While not illustrated in Figs. 2 or 3, a final separation stage may be provided immediately before the recovery furnace 26 to remove sodalite which has precipitated, in delayed fashion, during the concentration of the black liquor.

it will be understood that the aluminum ore, bauxi.e, may be substituted for pure aluminum oxide at the above indicated points of introduction. Indeed, this substitution of bauxite for pure aluminum oxide is preferred where suitable bauxite is available, because bauxite is relatively inexpensive and, as a consequence, the sodalite precipitate may be wasted rather than subjected to processing for recovery of reactant. The aluminum hydrate added, whether or not in the form of bauxite, is preferably in the form of the trihydrate.

There are certain consequences resulting from the point in the process at which the addition of aluminum oxide is made which ought to be considered. Thus, where the aluminum oxide addition is made to the digester either through line 100 or 9', the sodalite is retained in the pump exiting through line 20. Subsequently, the sodalite is separated from the pulp in the cleaning and/or bleaching steps to which the pulp is subjected. When raw bauxite is added to the digester 11 via line 100, any impurities in the bauxite must be removed from the pulp in the cleaning and bleaching steps.The addition via line 9', where the Al203 is dissolved from the bauxite before adding to the digester 11, is particularly designed for those cases in which the bauxite contains deleterious insolubles, such as very finely divided Fe203 which cannot be removed in the cleaning or bleaching steps. The reaction-dissolving station 91 is therefore followed in line 9' by the separator 92 in which Fe203 and/or other insoluble material is separated and removed through line 93. The clear white liquor containing the A1203 as Na20 Awl203 proceeds from the separator 92 to the digester 11. In this case, removal of the sodalite from the pulp may not be necessary because discoloring and other impurities have been removed in separator 92.

A preferred embodiment of the process of the invention is illustrated in Fig. 4 in which a batch digester 11 is shown with the various feedlines for carrying out the usual process of digestion, with provision for introducing bauxite into the digester to process a silica-contaminated feed. A silicacontaminated cellulosic fiber such as bagasse, bamboo or wheat straw is conveyed through the line 65 to a feed storage bin 66. The cellulosic fiber feed is introduced into digester 11 through line 3. White liquor from the causticizing stage of the process is introduced into a white liquor storage tank 51 through conduit 50. From the white liquor storage tank 51 the white liquor is routed to conduit 59 via conduit 9 in which pump 52 is located. Weak black liquor from the brown stock washers is introduced into the black liquor storage tank 56 through line 55.From the black liquor storage tank 56 the black liquor is routed to conduit 59 through conduit 7 in which pump 57 is located. Thus, the white liquor and black liquor flow in conduits 9 and 7, respectively, are joined in conduit 59 and the combined flow passes into the digester 11. Steam is introduced into the digester 11 through line 5 to provide the required reaction temperature and pressure. Inert and noncondensible gases are exhausted from the digester 11 through relief conduit 68. A bauxite storage bin 60 is provided which is filled through line 61. A conveyor 62 transports the suitably comminuted bauxite (preferably minus 65 or minus 100 mesh-Tyler Mesh series) to conduit 100 which directs the bauxite into the digester 11.

In the digester, at the reaction temperature and pressure, the cooking liquor attacks and dissolves the lignin in the cellulosic fiber material, while the bauxite dissolves and reacts with the silica dissolved from the cellulosic fiber material and with sodium hydroxide to produce a sodalite precipitate.

Following digestion the resulting mixture is routed to the blow tank through line 12.

For the purpose of iliustrating the advantages of the invention to those skilled in the art, the following examples are given: Examples-Procedure A depithed bagasse was obtained from a bagasse pulp mill. The bagasse contained 76.0 s moisture, and the ash from ignition at 5500C was 4.02% of the oven dried (O.D.) bagasse. 75.1% of the ash was SiO2.

Tests were conducted in a 4 liter autoclave, in which 1 80 grams of bagasse (O.D. basis) was cooked with soda white liquors In all cooks, 750 grams of moist bagasse was treated with 306 ml of white liquor which contained 71.3 gpi of NaOH and 8.6 gpl of Na2CO3.500 ml of water was added to the digester. The cooking cycle was 4 minutes to temperature and pressure (171 0C, 110 psig), ten minutes cooking, and 6 minutes to cool to 1000C. The cooked pulp was then filtered and washed and the black liquor and pulp were analyzed.

Control Sample In this case, the silica-contaminated bagasse was cooked as described above to determine the amount of silica present in the dissolved solids of the black liquor. It was determined that the black liquor contained 6.85% total dissolved solids and of this total, silica made up 0.1 70796, or 2.5% of the total dissolved solids. The 1 350 grams of black liquor produced by cooking the bagasse contained 2.3 grams of soluble silica. This amount of silica in the black liquor is enough to cause serious scaling problems in the evaporation step, objectionable increase in the viscosity of the black liquor and substantial loss of lime in the recausticizing step.

Example I in a test simulating a process in which bauxite is added to white liquor (see line 90, Fig. 1), the white liquor is clarified to remove residual bauxite and then pumped to the digester to react with the soluble silica, 4.38 grams of hydrated sodium aluminate is dissolved in the white liquor. The white liquor contained 1.95 grams Awl203, which is stoichiometrically equivalent to 2.29 grams SiO2. Using the cooking cycle and washing previously described, the black liquor has 7.06% total dissolved solids, which included 0.011% SiO2 and 0.063% Al203. The treatment has been effective in that the silica has been reduced to 0.16% of the total dissolved solids from the value of 2.5% obtained in the control sample.

A slightly lower dose of Awl203 is also effective. For example, using the stoichiometric proportion of 0.0243% A1203,the SiO2 is 0.0286%.

Example II In a test simulating a process in which bauxite is added to the digester (see line 100, Fig. 1), 3.5 grams of minus 200 mesh bauxite is added to the white liquor just before the liquor is added to the bagasse in the digester. The bauxite contained 80.23% A120s and 2.75% SiO2 (15.03% is lost on ignition at 6000C). The bauxite contained 2.81 grams Awl203, which is 22% more than required to react with the silica in the bagasse (neglecting SiO2 in bauxite}. The 6.7% total dissolved solids black liquor contains 0.056% SiO2, and the excess Al203 is very low, only 0.0064%. The 10 minutes digestion time is not long enough to dissolve sufficient Awl203 to react with all of the SiO2 present; consequently the SiO2 is only reduced to 0.056% in the black liquor.

In order to obtain more complete reaction with the silica, a more finely ground bauxite, or additional minus 200 mesh bauxite could be added to the digester, to put more Al203 in solution, or some AT203 could be added to the white liquor as was done in Example I.

Example Ill 5.3 grams of minus 65 mesh bauxite is added to the white liquor immediately before the liquor is added to the bagasse in the digester. The bauxite is the same composition as in the preceding test. The bauxite contains 4.25 grams Al203, which is 85% more than required to react with the silica in the bagasse (neglecting the SiO2 in bauxite). The 6.6% total dissolved solids black liquor contained 0.0277% SiO2 and 0.03019/0 Awl203. The bauxite dosage is thus seen to be slightly excessive. For a stoichiometric ratio of Al203 to SiO2, the Awl203 shouid be 0.0266% and SiO2 0.0312% in the black liquor.

Example IV 5.6 grams of minus 65 mesh is added to the white liquor charged to the digester. The 6.25% total dissolved solids black liquor contains only 0.0161% SiO2 and the Awl203 is up to 0.0606%.

The above examples conclusively demonstrate that desilication of silicon-contaminated pulp liquor can be achieved simply and economically by the addition of aluminum oxide in the form of hydrated aluminum oxide and, particularly, the naturally-occurring aluminum ore, bauxite.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended

Claims (13)

claims. Claims

1. A process for desilicating silica-contaminated pulp liquor which comprises introducing a quantity of an aluminum oxide selected from the group consisting of the hydrated aluminum oxides into the liquor to precipitate silica as a sodalite.

2. The process of claim 1 wherein the hydrated aluminum oxide addition is made in the form of the trihydrate.

3. The process of claim 1 or claim 2 wherein the hydrated aluminum oxide addition is made in the form of bauxite.

4. A process for desilicating a silica-contaminated pulp liquor produced in the alkaline pulping process, said alkaline pulping process including the steps of digesting cellulosic fiber material with a sodium hydroxide-containing white liquor, separating the resulting black liquor from the pulp, concentrating the black liquor in multi-effect evaporators, combusting the concentrated black liquor to yield black ash, smelt or pellets and treating the black ash smelt or pellets to recover sodium hydroxide in the form of white liquor which is recycled for the digestion step; the improvement comprising introducing an aluminum oxide selected from the group consisting of the hydrated aluminum oxides into the pulp liquor to precipitate silica as a sodalite.

5. The process of claim 4 wherein the aluminum oxide addition is made in the digestion step.

6. The process of claim 5 wherein the aluminum oxide addition is made in the form of finely divided bauxite.

7. The process of claim 4 wherein the aluminum oxide addition is made to the white liquor to produce the intermediate compound sodium aluminate for reaction with the silica.

8. The process of claim 7 wherein the aluminum oxide addition to the white liquor is made in the form of finely divided bauxite.

9. The process of claim 8 wherein, following the reaction of the bauxite ore and the white liquor, solid impurities introduced with the bauxite ore are separated and removed from the process, and the white liquor-sodium aluminate solution is forwarded for the digesting step.

10. The process of claim 9 wherein the aluminum oxide contained in the bauxite ore is in the form of the trihydrate.

11. The process of claim 4 wherein the aluminum oxide addition is made to the black liquor prior to the concentration step.

12. The process of claim 4 wherein the aluminum oxide addition is made to the black liquor in an intermediate stage of evaporation.

13. The process according to claim 11 or claim 12 wherein the aluminum oxide addition is made in the form of bauxite ore of the trihydrate type.