CA1193406A - Process to regenerate kraft liquor - Google Patents

Abstract

This is a process for regenerating white liquor from residual kraft liquors. The process comprises reacting at least one metal oxide member selected from ferric oxide and/or titanium dioxide with a sodium carbonate product derived from a residual kraft black liquor under vigourous oxidation condition that is near but below the eutectic temperature of the derived product and of the selected member, to form particles comprising a mixture of sodium sulfate and of at least one sodium member of the class consisting of sodium ferrite and sodium titanate, reducing the particles in solid phase by converting the sodium sulfate in the particles to sodium sulfide in the presence of a reducing gas. The reduced particles are then dissolved in an aqueous medium to form a white liquor, and to convert the sodium member into sodium hydroxide soluble in the white liquor and into an insoluble metal oxide of the selected member. The insoluble metal oxide is separated leaving the white liquor composed of dissolved sodium sulfide and sodium hydroxide in an aqueous solution.

Description

FIELD OF THE INVENTION
This invention relates to a process operated in solid phase for the recovery of chemicals and energy from kraft pulping liquor. More specifically the present invention relates to a process using unconventional caus-ticizing agents in the recovery of kraft liquor and special rigourous oxidation conditions, and wherein causticizing with lime as well as the handling of molten smelt, lime kiln and its auxiliary equipment are elimi-nated.
BACKGROUND OF THE INVENTION
Ar~rlarticle in Paperi ja Puu - Papper och No. 3 1978, p. 129, summarizes in a discussion, the possibil-ities of reducing the capital costs of new pulp mills in applying non-conventional techniques to the recovery of spent pulping liquors. Autocausticizing is one of the techniques and is based on the well known facts that certain amphoteric oxides react with sodium carbonate, liberating the alkali compound carbon dioxide from the mixture at high temperatures, and forming mixed oxide compounds. When dissolved in water, the compounds formed in this manner produce sodium hydroxide. The article briefly reviews through patents and publications the proposed recovery systems of the recent years which are broken down into two groups. The first deals with the methods intended to eliminate the conventional recovery furnace and hence the associated smelt explosion hazards. These include wet combustion, partial wet combustion, fluidized bed, hydropyrolis and SCA - Billerud

- 2 -method:.. 'fhe secant! group de<-s.ls with the melho~is designed to e.iimi.nate the cau,ticiz.ing and line kiln systems.
The,e include Frametsa's proposal in which black liquor combustion is carefully controlled and some other methods in whit h amphoteric oxides are used to liberate carbon dioxide from the smelt.
It has also been proposed in US patent No. 4,000, 264 issued December 28, 1976 as invented by Nagano et al, to burn spent soda liquor which is generally free of sulfur, in the presence of ferric oxide (Fe203) to obtain a smelted product containing sodium ferrite (Na2Fe204) which, when hydrolyzed, gives a solution of NaOH and insoluble Fe203. The former returns as pulping liquor of the soda process, and the latter is used again in the burning of additional spent liquor.
For spent liquor derived from the sulfite process and which contains sulfur, Canadian patent Pdo. 590,226 issued January 5, 1960 as invented by Alenko, also proposed to use Fe203 during the liquor burning opera-tion. The product obtained is a mixture of Na2 Fe204 and Fe3S2, which when hydrolyzed gives a solution of NaOt1 and a residue of Fe203 and Fe3S2. The roasting of the residue then transforms its Fe3S2 fraction into Fe203 and So2 gas. A subsequent reaction between S02 gas and NaOH solu-tion will produce a liquor of sodium sulfite (Na2S03) to be used in the pulping operation of the sulfite process.
Similarly, regenerated Fe203 is reused in the spent liquor burning operation.
Tn kraft process, after, its use in pulping opera-

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tion, l:hc~ wlri tc~ 1 iclrior, whtich i~: <t sc>lut:iort of N<'r()fl ,tncl Na2S, I~oco~rr~~; l Iti~ s;p~nt 1 icluor, which ir:; nnrmal.Ly hurnt in ;r recovery lu~i Ic~r fnrn<~cc~ to generate steam and a molten smelt of C1a2C03 and Na2S. 'fo convert the resultant Na2C03 in the smelt ineo N,rOll, ~e.rman patent No. 2257463 as invented by Kato et al or. British patent No. 1,407,276 proposes to mix titanium oxide (Ti02) or illmenite (a mixture of Fe203 and Ti02 with a trace of Si02 and A1203) with the smelt in a special furnace and in an inert environment. When only Ti02 is used, the product leaving the special furnace is a molten mass of Na20.Ti02, Ti and Na2S. When illmenite is used, the molten mass also con-tains Na20.Fe203 and Fe2S3. During subsequent hydrolysis, Na2Fe204 and Na20.Ti02 become NaOH in solution and settle-able Fe20 and Ti02. Fe2S3, formed as a result of the reaction between Na2S and Fe203, is also insoluble and separated from the NaOH solution together with Fe203 and Ti02. However, the formation of insoluble Fe2S3, is at the expense of Na2S which may completely disappear from the NaOH solution produced and the regenerated white liquor is depleted in Na2S and is therefore not the desir-able end product.
Other means have been taught Eor regenerating white liquor, for instance Canadian patent 828,654 issued December 2, 1969, as invented by Arnold, discloses a process Eor the regeneration of an alkaline white liquor.
that is suitable for the soda pulping process, from a spent black liquor resulting Erom such a soda pulping process, which comprises concerttrat.ing the black Liquor,

- 4 -mixing the concentrated black liquor with calcium carbon-ate or calcium oxide, oxidizing the concentrated black liquor mixture at a temperature between approximately 693°
and 816°C to completely burn the combustible organic components and form a product containing sodium carbonate, calcium oxide and sodium sulfate, subjecting the product to a temperature of about 649° and 704°C in the pres~;nc~-of hydrogen to convert the sodium sulfate to sodiu~:~
sulfide, and passing the resulting product into water and separating the precipitated calcium carbona~Ee from the resulting slurry, leaving a solution containing sodium sulfide and~Isodium hydroxide which is a regenerated white liquor that is suitable for use in the production of pulp in accordance with the soda process.
To summarize the prior art before the present invention, 'the use of Fe203 and/or Ti02 in the recovery of spent pulping liquors was restricted as to the types of liquor regenerated and/or as to the methods of applica-tion: For soda liquor or sulfite liquor, Fe2o3 and/or Ti.o2 could be directly added to a recovery furnace where smelt is obtained. For kraft liquor, however to regener-ate the spent liquor in the desirable composition i.e. a solution of Na2S and NaOH, only TiOZ could be used, and it _ could only be added into the smelt leaving the recovery furnace, and an inert environment, that is one which is neither oxidizing and nor reducing and which had to be maintained throughout the fusion reaction between Na2C03 and Ti02.
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BRIEF UGSCRIL"PION Of~ 'fCIE; Plt~;if':N'P INVC;N'PION
It is an object of the present invention to over-come the problems facing kraft liquor recovery underlined herein below.
S It is also object of the present invention to provide a new process for recovering a white liquor from residual kraft liquor without going through the conven-tional melting route and without using line kiln.
Broadly stated the present invention is directed to a process for regenerating white liquor from residual kraft liquor, said process comprising: reacting at least one metal .oxide member selected from the class consisting ferric oxide and/or titanium dioxide with a sodium carbon-ate product derived from residual kraft black liquor, under rigourous oxidation condition, near but below the eutectic temperature of said derived product and of said selected member, to form particles comprising a mixture of sodium sulfate and of at least one sodium member of the class consisting of sodium ferrite and sodium titanate, reducing said particles in solid phase by converting said sodium sulfate in said particles to sodium sulfide using a reducing gas, dissolving said reduced particles in an aqueous medium to form a white liquor, and to convert said sodium member into sodium hydroxide soluble in said white liquor and into an insoluble metal oxide of said selected member, separating said insoluble metal oxide, said white liquor being composed of dissolved sodium sulfide, and sodium hydroxide in an aqueous solution.
_ f, _ ~.~.~,'.3~f~6 l3Ct(.I:E' Dt~,SCRIE"fION Of' 'fEif3 I)CtAWINGS
(~ur-tl~er. featuces, objects and advantages will. be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which:
Figure 1 is a schematic illustration of an embodiment of the present invention, Figure la is a schematic illustration of another embodi-ment of the present invention, Figure 2 is a graph illustrating the relationship between carbon dioxide removal efficiency and pellet size, Figure 3 is~ graph illustrating the relationship between reduction efficiency and reducing time, and Figure 4 is another graph illustrating the relationship between reduction efficiency and reduction temperature.
DESCRIPTION OF TtiE PREFERRED EMBODIMENTS
As illustrated in Figure 1, slurries metallic oxide (Mt0) such as titanium dioxide and/or ferric oxide pumped into line 10 (from tank 11 or line 99) is sprayed directly into a fluidized bed recovery furnace 12, or alternatively via 15 to be mixed with the residual kraft liquor (black liquor) which in turn is fed via line 18 to the fluidized bed unit 12 where the liquor is oxidized (burned) using air entering via line 19. The temperature of the bed is partly controlled by water which enters the furnace via line 20 and steam which is produced therefrom and which leaves the .furnace via line 21.
Alternatively the metallic oxide may be added directly to the fluidizec9 hed in solid form, for instance ~~.J~~O~i as partic.lcs, and ~i por.tion directly to the .liquor so that the total metallic oxide added is ac9ded via both routes.
The amount of metallic oxide that may be added to the black .Liquor is generally from about 1 to about 2 mole per mole of Na2C03 present in the black liquor and preferably about 2:1. Its point of addition is limited by operating constraints such as consistency, viscosity, spray patterns, etc., of the black liquor. A too high consis-tency results in a high liquor viscosity and interferes with the spray operation.
In the fluidized bed 12 the organic material in the black i,iquor is burned to generate energy and pellets of inorganic material consisting primarily of a mixture of sodium sulfate and sodium ferrite and/or titan-ate are formed. The temperature in the bed 12 approaches the eutectic temperature (in the order of about 850-lODO°C) which leads to pellet formation and conversion of sodium carbonate to sodium titanate and/or ferrite. The presence of metallic oxides is essential and enables the oxidation process to be carried out under vigourous conditions.
Without them, the eutectic temperature of the liquor inorganics would be in the order of 670-700°C and the process to liberate C02 from Na2C03 cannot take place. On the average, the residence time of solids in the bed is over about half a day. Preferably heat is recovered via the line 21 from the bed and used to generate steam for example. Generally the bed is very large to provide for the residence period of half a day, and has means such as screw conveyors to continuously remove pellets from the bed at the out:.l~t: l2(1. from the out.lnt: 12f~, thc~ peL.lets are thcu c3i.rected via line 22 to a reduction Furnace 24.
In said furnace 24, these pel:Lets are reduced in solid phase, and the spent reducing gas carried via line 26 to a suitable boiler to recover heat, or to the fluidized bed recovery furnace 12 as illustrated, or alternatively as illustrated, is recycled via line 26A regenerator 27 and line 25A to reenter with the fresh reducing gas in line 25.
The reduction furnace 24 (preferably a fluid bed) is generallx operated under atmospheric or pressurized condition,~at elevated temperatures of about 500°C to 700°C, bearing in mind the reduction efficiency is better at higher temperature, but that the temperature must not be so high as to melt the pellets.
Time should be in the order of a few hours, depending upon the efFectiveness of the reducing furnace, and may be even less than 1 hour at 600° under ideal conditions.
The reducing gas entering the furnace via line 25 is preferably hydrogen which is more effective in the solid phase reduction of sodium sulfate to sodium sulfide with either sodium ferrite or sodium titanate present.
Also, if desired, ~ mixture of hydrogen and carbon monoxide, and natural gas may be used when the metal oxide is titanium dioxide.
As above indicated, if desired, the spent reduc-ing gas being regenerated may be recycled. In the case of It2, water vapour is the only undesi.rec) product and it may <~ _ :~,1;~:3~t~~
be condensed. In such a case the regenerator 27 becomes simply a water condensor used toefore recycling the t12. If.
a mixture of Ev2 and CO are used, hoth C:02 and H20 have to be separated in the regenerator before returning the spent reducing gas to the reduction furnace.
The reduced ferrite and/or titanate pellets which have been maintained is solid phase throughout the above recovery steps, leave the reducing furnace via line 28 and are dissolved in the dissolving tank 30. In this tank the hydrolysis of the sodium titanate and/or ferrite produces sodium hydroxide and metal oxide such as titanium dioxide and/or fer,rlic oxide. The metal oxide precipitates and is separated in the separation unit 32. The white liquor (dissolved Na2S and NaOH) then leaves the separation unit 32 via line 34 where it may be returned to the pulping operation.
The separated metal oxide slurry (called dregs) is conveyed via line 36 to a drier 38 in which water is evaporated preferably by the waste energy present in the flue gas, (from the furnace 12 via line 40), which are subsequently exhausted from the drier 38 and fed to the stack gases, via line 42. The dried metallic oxide material is then used together with make up oxides and fed - to line 10 via line 44.
It may be desirable to bleed some of the dregs from the separator from the separation unit via line 46 in the event there is a build up of inactive materials in the system.
Instead of adding metal oxide in the Furnace ~.~J~~Cf>
where I:iac, IW~c:k liquor is burned, in E'icy.rrc: la, tire nu~tal oxide entcrr:, ~r mixer 46 via line 4~. While oxidized black liduor solids. preferably pellets from a (luidized bed, comprising sodium sulfate and sodium carbonate, enter via line 50. The metal oxide and solid particles of sodium carbonate and sodium sulfate are mixed in the mixer 46 and pass via line 52 into a roasting furnace 54 that is preferably a rotary type furnace. Tf desired, depending on the design of the furnace the mixer and furnace may he combined and the metal oxide and the particles of sodium sulfate and sodium carbonate may be mixed in the furnace 54 itself.,. This furnace is operated at a temperature in the order of 900°C to react the sodium carbonate with ferric oxide and/or titanium oxide to form sodium ferrite and/or sodium titanate as occurs in the f.luidized bed 12 of the Figure 1 embodiment.
The ferrite or titanate particles leave the furnace 59 via line 22A and pass to the reduction furnace 24 while the spent reducing gases may be returned via line 26A to the furnace 59, or recycled to the reduction furnace as above described. Combustion air enters the ferrite or titar?ate furnace via line 2oA and the hot flue gases leave via line 40A and may be used in the manner described herein above with respect to the flue gases from the recovery furnace.
The following examples .serve to illustrate particular aspects of the invention.
Example 1 Samples of kraft lic7uor inorganics containing -~..~.:.~L$~~øi about_ 65'k No2Cn3 ~nci a!>Y, N~ySOq in the (orm of pellets of various sizes and Fe203 in the Corm of: powc9er are mixed and roastec7 in a muffle furnace at a temperature of about fi25°C. A mole ratio of 1 to 1 of: Fe203 over Na2C03 is used in all experimental runs. The reduction in the sample weight, which is observed only for samples contain-ing Fe203, clearly indicates the formation of sodium ferrite (PIa20.Fe203) together with the release of C02 from the samples. Figure 3 summarizes all the results obtain-ed. The effect of pellet size and roasting period is self explanatory:. a smaller pellet size and a longer roasting period will result in a better C02 removal efficiency.
In all cases, when pellet samples are left for a period of about 20 hours, the removal of C02 from the tJa2C03 fraction is substantially complete, for all the particle sizes tested.
Example 2 In example 2, tests similar to the ones described in example 1 were carried out. Elowever, instead of Fe203, the amphoteric oxide used is either Tio2 in concentrated, rutile or reagent grade Ti02. The resultant samples which contain sodium titanate, are then dissolved in hot water to obtain a solution consisting of NaOEi and Na2SOq and insoluble Ti02. The results summarized in Table 1 clearly indicate that a higher Ti02 dosage and a longer roasting time will result in a better conversion of Na2C03 into Na0El.

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'fable l clearly i L W strates that in the case of '1i02, at x)00°C, to obtain a substantially complete conv~r-sion of NaZC.C)3 present in kraf t liquor inorganics into sodium titanate, and ultimately into NaOH, a mole ratio Ti02/Na2C03 of up to 2.0 and a time of .10-20 hrs are required.
On the average the residence time of the kraft liquor inorganics in the fluidized bed recovery furnace, is generally well over 12 hours, adequate conversion of sodium carbonate to sodium ferrite or titanate will occur provided ample amounts of metal oxides are present.
Similarly with respect to the embodiment discussed and shown in Figure 2, the residence time of pellets in the furnace 54 must be sufficiently long in order to obtain the desired degree of conversion of their sodium carbonate fraction into the desired sodium titanate and/or ferrite.
Examples 3 and 4 Example 3 is a series of exaerimental runs used to illustrate the efficiency of the reduction stage used to reduce the Na2SOq fraction present in the liquor inorganics into Na2S.
In these experiments, mixtures of kraft liduor inorganics and Fe203, after being roasted in a muffle furnace at about 800°C for a period of about 17 hours, are reduced with hydrogen. The samples obtained are then dissolved in 90°C water for a period of about 1 hour to give a solution containing Na20Ei, Na2S and Na2S04 and insoluble Fe203~
Figure 3 summarizes the results obtained when a reducing temh5er.ature of 500°C and a rrmlucind time varying from 0.5 hours to 3 hours were used.
Similarly, figure 4 shows the cEEect of reduction temperature on reduction efficiency when a reducing time of 1.0 hours was used for all samples.
Example 5 Tests similar to the ones described in Examples 3 and 4 were carried out. However, instead of Fe203 and H2, the causticizing and reducing agents for this case are Ti02 and a mixture of 35$ CO and 65$ H2 respectively.
Similarly, a roasting furnace temperature of 900°C and a roasting time of about 17 hours were used to prepare the samples for the reduction experiments. A reducing temper-ature varying from 525°C to 600°C and a reducing time ranging from 1.0 hour to 4.0 hours were then used. The dissolving of the samples as prepared give the results summarized in Table 2.

EFFICIENCY ($) -REDUCING REDUCING
TEMPERATURE TIME CAUSTICIZTNG (1) REDUCTION (2) (°C) (HRS) 525 2.0 91 100 500 2.0 Q9 72 500 4.0 65 100 600 1.0 93 93 (.l) Moles Na2C03 converted into NaUn per initial moles Na2C03 in sample (2) .Moles Na2S formed per moles Na2SOq in sample Having disclosed the invention modifications will be evident to those skilled in the art without departing ~.~w~~~:~~~
Crom the spirit of tile invention ~, clef fined in the appendix claims.

Claims (10)

1. A process for regenerating white liquor from residual kraft liquors, said process comprising: reacting at least one metal oxide member selected from the class consisting ferric oxide and/or titanium dioxide with a sodium carbonate product derived from said residual kraft black liquor under rigourous oxidation condition, near but below the eutectic temperature of said derived product and of said selected member to form particles comprising a mixture of sodium sulfate and of at least one sodium member of the class consisting of sodium ferrite and sodium titanate, reducing said particles in solid phase by converting said sodium sulfate in said particles to sodium sulfide using a reducing gas, dissolving said reduced particles in an aqueous medium to form a white liquor, and to convert said sodium member into sodium hydroxide soluble in said white liquor and into an insoluble metal oxide of said selected member, separating said insoluble metal oxide, said white liquor being composed of dissolved sodium sulfide, and sodium hydroxide in an aqueous solution.

2. The process as defined in claim 1, wherein a combustible portion of a residual black liquor is burned in the presence of said selected metal oxide and of oxidizing gases to form residual kraft particles to be fluidized in solid phase and to react under oxidation condition with said metal oxide to form particles compris-ing a mixture of sodium sulfate and of at least one sodium member of the class consisting of sodium ferrite and sodium titanate, said particles being fluidized by said oxidizing gases wherein the total amount of said metallic oxide from about 1 to about 2 mole per mole of Na2CO3 present in said residual black liquor and wherein the temperature of said fluidized particles is maintained at a temperature near, but below the eutectic temperature of said particles forming said bed, by adjusting the tempera-ture, concentration and feeding rate of said residual black liquor of said metal oxide and of said oxidizing gases, and said particles so oxidized are then reduced.

3. The process as defined in claim 1, wherein said metal oxide is reacted with particles containing sodium sulfate and sodium carbonate in a roasting furnace to form said particles containing said sodium ferrite.

4. The process as defined in claim 1, wherein said metal oxide is ferric oxide.

5. The process as defined in claim 1, wherein said metal oxide is illemite.

6. The process as defined in claim 1, wherein the effective reducing component of said reducing gas is hydrogen.

7. A process as defined in claim 1, wherein said reducing gas comprises primarily hydrogen and carbon monoxide and said metal oxide is titanium dioxide.

8. The process as defined in claim 1, wherein said precipitated oxide is recycled to react with further of said sodium carbonate derived from further burning of said spent liquor.

9. The process according to claim 1, wherein said particles are maintained at a temperature between 850° and 1000°C but below eutectic temperature for a residence period of about half a day.

10. The process as defined in claim 1, wherein said particles are reduced in solid phase at a temperature between 500°C and 700°C, but below the melting point of said particles.