Claims:
1.A process for weak black liquor conditioning to scale minimization in Kraft MEEs, comprises the steps of ;
a) allowing reaction of raw green liquor 100-140 g/L as Na2O with lime up to 78-80% causticity in slacker-1;
b) transferring slurry obtained in step a to WLCD/WLC-1 (4,5) through Causticizers unit (2) and feed pump(3);
c) filtering out strong WWL stored in a strong WWL tank(19) and fed to Slacker (7) by using pump (20) through a control valve and flow meter;
d) allowing reaction of Filtrate/Clear White liquor(6) from WLCD /WLC-1(4,5)(Clear White liquor) of pH14 and temp 85 to 90 Deg C,100-140 g/l with CaO (75-95% Purity) (17)to react with remaining Na2CO3 till 90-95% Conversion in slacker no.2(7);
e) feeding slurry obtained in step d to a causticizer units (8) having retention time of 60-180 mins and having temp-100 deg C then to WLC-2 (10) by feed pump (9) and Clear white liquor of WLC-2(10) or partially turbid come to White liquor feed tank(21) is fed to 20-40 micron filter(23) with the help of a high head pump(22) and to slacker(1) with the help of pump(13) and Line (14)
f) storing Clear White liquor from 20-40 micron filter(23) in WL supply tank (24) and further supplied to pulp mills as of requirement;
g) feeding rejected mud to WLC-2 discharged mud funnel (11) system in control flow;
h) scrapping of lime mud by WLCD/WLC-1(4,5) and further diluting it with WWL (chute shower);
i) feeding slurry obtained to Cylindrical mesh stocking filter (18)(poly propylene 20-40 micron or SS 100 mesh) with the help of high head pump (greater than 50 Mtr)(26);
j) storing filtrate obtained from filter (18) in separate tank (19) and mixing it in slacker (7) or (21) in control way using a magnetic flow meter as per requirement ; and
k) feeding high density mud obtained by filter (18) 1300-1500 kg/m3 in a separate mud mixing tank for further washing.
2. The process as claimed in Claim 1, wherein 50% calcium attached (500g -1000g/ton wood)with raw material can be removed by washing the raw material obtained above with weak green liquor or diluted soda ash (Na2CO3) as per reaction below.
Na2CO3 +CaO & Ca ions (raw material calcium) + H2O CaCO3 +2 NaOH
And followed by further as described above in process details (Part-B: This step will be done in pulp mill).

Description:
FIELD OF THE INVENTION

[0001] The present invention relates to processes Kraft MEEs, in particular the present invention relates to process for weak black liquor conditioning to scale minimization in Kraft MEEs by employing 95% Causticity in two stages and raw material washing with weak green liquor.

BACKGROUND OF THE INVENTION

[0002] Kraft multiple effect is one of the most important equipment to process the spent cooking (weak black liquor). Kraft MEE are also called heart of the Soda chemical recovery process department and it is really true. If MEEs are running good one can notice remaining department performance itself runs goods i.e Smelt reduction efficiency is good, sulphidity is optimum ,Recovery boiler steam generation is good, Spouts and airports jamming is minimum, Lesser dregs content in green liquor hence produced which in turn more white liquor generation and so many.

[0003] Unfortunately due to some ignored or misoperation, heating areas of MEEs gets severely plugged with different types of scale and if not timely water or weak white liquor boiled, these scale increases day by day that one day one has to bypass individual effect for hydojetting. Which impact overall performance of Soda chemical recovery department if additional standby effect is not there. Because Kraft MEEs are not only energy intensive equipment but also the capital intensive equipments and every dollar invested on equipment will depreciate as well as interest also to be paid off for installing those.

[0004] It has been observed that MEEs are plugged from VIIth effect to IIIrd effect by pulp(fibers) and lignin and sludge especially Falling film lamellas technology. Investigation shows that almost in this area scale contains 50-60% organic , 40-50% inorganic and other calcium based . IInd effect found scaled up with some a hard scale which bit of Calcium carbonate, lignin sludge and silica. Toughest scale are found in the finishers and super concentrator, in Finisher scale are basically of a. Silica and Calcium carbonate b. burkeite (2mole Na2SO4 and 1Mole Na2CO3) c. dicarbonate (2Mole Na2CO3 and 1 mole Na2SO4). Up to critical solids 50-55% no such scale is observed (drawing.3). Also note that if Mass ratio of Na2CO3/Na2SO4 is up to .2 only Na2SO4 will crystallize and if this ratio increases up 1.5 only Burkeite (1Na2CO3+2Na2SO4) is formed, Burkeite scale can be removed by hot water wash or weak white liquor or weak black boiling, and if this ratio increases beyond 1.5 to 2.0 Dicarbonate (2 Na2CO3+1Na2SO4) and Burkeite will be formed, If ratio increase more than 2.0 only dicarbonate will be formed at solid concentration 67% or above. Dicarbonate scale are really hard and difficult to remove from crystallizers, till now no such full proof chemical aid or anything found out to inhibit these scales albeit many companies of chemical dosing claims to be. Three notorious constituents namely calcium and silica, aluminum content are the hardest scale contributor and these are inherent part of raw material and one must ferret out some way to remove these before they clump up as hardest scale in Finishers and Super concentrator heat transfer areas.

[0005] EP1664421B1 relates to a process for removing silica from nonwood plant materials involving both chemical and mechanical action is described. The silica-rich epidermal cells are liberated mechanically by a pre-pulping and low-consistency refining step and subsequently removed from the pulp via the filtrate of a thickening step. Amorphous silica is liberated chemically by using an alkaline dilution source in the pulper, then removed from the pulp via the filtrate of a thickening step and a dewatering step. The silica is then removed from the filtrate by adjusting the pH, followed by a separation step. The desilicated fibrous material may then be chemically or mechanically pulped and bleached using known processes. The silica removed from the wood & nonwood plant material may then be used as a feedstock for other applications.

[0006] Hence, there is need to develop a process for weak black liquor conditioning to scale minimization in Kraft MEEs by employing 95% Causticity and raw material washing with weak green liquor.

OBJECTS OF THE INVENTION

[0007] The principal object of the present invention is to overcome the disadvantages of the prior art.

[0008] An object of the invention is to provide a process for weak black liquor conditioning to scale minimization in Kraft MEEs by employing 95% Causticity and raw material washing with weak green liquor.

[0009] Another object of the invention is to provide a process that reduces silica content from WBL, reducing weight ratio of Na2CO3 content in white liquor and in turn increases weight ratio of Na2SO4 in WBL. i.e. (Na2SO4)/(Na2SO4+Na2CO3)

[0010] Yet another object of the invention is to provide a process that reduces acid insoluble in final pulp, reduces alumina from system to reduce aluminum silicate scale and reduces NaCl from chemical recovery cycle.

[0011] The objects of the present invention will become readily apparent upon further review of the following detailed description of the preferred embodiment as illustrated in the accompanying drawings.

SUMMARY OF THE INVENTION

[0012] The present invention is directed to a process for weak black liquor conditioning to scale minimization in Kraft MEEs by employing 95% Causticity and raw material washing with weak green liquor.

[0013] According to an embodiment of the present invention, the process for weak black liquor conditioning to scale minimization in Kraft MEEs, comprises the steps of ;

[0014] allowing reaction of raw green liquor 100-140 g/L as Na2O with lime up to 78-80% causticity in slacker-1;

[0015] transferring slurry obtained in step a to WLCD/WLC-1 (4,5) through Causticizers unit (2) and feed pump(3);

[0016] filtering out strong WWL stored in a strong WWL tank(19) and fed to Slacker (7) by using pump (20) through a control valve and flow meter;

[0017] allowing reaction of Filtrate/Clear White liquor(6) from WLCD /WLC-1(4,5)(Clear White liquor) of pH14 and temp 85 to 90 Deg C,100-140 g/l with CaO (75-95% Purity) (17)to react with remaining Na2CO3 till 90-95% Conversion in slacker no.2(7);

[0018] feeding slurry obtained in step d to a causticizer units (8) having retention time of 60-180 mins and having temp-100 deg C then to WLC-2 (10) by feed pump (9) and Clear white liquor of WLC-2(10) or partially turbid come to White liquor feed tank(21) is fed to 20-40 micron filter(23) with the help of a high head pump(22) and retained free lime sludge to slacker(1) with the help of pump(13) and Line (14)

[0019] storing Clear White liquor from 20-40 micron filter(23) in WL supply tank (24) and further supplied to pulp mills as of requirement;

[0020] feeding rejected mud to WLC-2 discharged mud funnel (11) system in control flow finally to slacker(1);
[0021] scrapping of lime mud by WLCD/WLC-1(4,5) and further diluting it with WWL (chute shower);

[0022] feeding slurry obtained to Cylindrical mesh stocking filter (18)(poly propylene 20-40 micron or SS 100 mesh) with the help of high head pump (greater than 50 Mtr)(26);

[0023] storing filtrate obtained from filter (18) in separate tank (19) and mixing it in slacker (7) in control way using a magnetic flow meter; and

[0024] feeding high density mud obtained by filter (18) 1300-1500 kg/m3 in a separate mud mixing tank for further washing.

[0025] Additional aspects, advantages, features and objects of the present disclosure would be made apparent from the drawings and the detailed description of the illustrative embodiments construed in conjunction with the appended claims that follow.

[0026] It will be appreciated that features of the present disclosure are susceptible to being combined in various combinations without departing from the scope of the present disclosure as defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other features, aspects, and advantages of the present invention will become better understood

[0028] with regard to the following description, appended claims, and accompanying drawings where:

[0029] Figure 1 illustrates the schematic diagram of the present process.
DETAILED DESCRIPTION OF THE INVENTION

[0030] As required, detailed embodiments of the present disclosure are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the disclosure which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure.

[0031] Various other objects, advantages, and features of the disclosure will become more readily apparent to those skilled in the art from the following detailed description when read in conjunction with the accompanying drawing.

[0032] In any embodiment described herein, the open-ended terms "comprising," "comprises,” and the like (which are synonymous with "including," "having” and "characterized by") may be replaced by the respective partially closed phrases "consisting essentially of," consists essentially of," and the like or the respective closed phrases "consisting of," "consists of, the like.

[0033] As used herein, the singular forms “a,” “an,” and “the” designate both the singular and the plural, unless expressly stated to designate the singular only.

[0034] The present invention is directed to a process for weak black liquor conditioning to scale minimization in Kraft MEEs by employing 95% Causticity and raw material washing with weak green liquor.

[0035] The process for weak black liquor conditioning to scale minimization in Kraft MEEs, comprises the steps of ;
[0036] allowing reaction of raw green liquor 100-140 g/L as Na2O with lime up to 78-80% causticity in slacker-1;

[0037] transferring slurry obtained in step a to WLCD/WLC-1 (4,5) through Causticizers unit (2) and feed pump(3);

[0038] filtering out strong WWL stored in a strong WWL tank(19) and fed to Slacker (7) by using pump (20) through a control valve and flow meter;

[0039] allowing reaction of Filtrate/Clear White liquor(6) from WLCD /WLC-1(4,5)(Clear White liquor) of pH14 and temp 85 to 90 Deg C,100-140 g/l with CaO (75-95% Purity) (17)to react with remaining Na2CO3 till 90-95% Conversion in slacker no.2(7);

[0040] feeding slurry obtained in step d to a causticizer units (8) having retention time of 60-180 mins and having temp-100 deg C then to WLC-2 (10) by feed pump (9) and Clear white liquor of WLC-2(10) or partially turbid come to White liquor feed tank(21) is fed to 20-40 micron filter(23) with the help of a high head pump(22) and to slacker(1) with the help of pump(13) and Line (14)

[0041] storing Clear White liquor from 20-40 micron filter(23) in WL supply tank (24) and further supplied to pulp mills as of requirement;

[0042] feeding rejected mud to WLC-2 discharged mud funnel (11) system in control flow finally to slacker to slacker1;

[0043] scrapping of lime mud by WLCD/WLC-1(4,5) and further diluting it with WWL (chute shower);

[0044] feeding slurry obtained to Cylindrical mesh stocking filter (18)(poly propylene 20-40 micron or SS 100 mesh) with the help of high head pump (greater than 50 Mtr)(26);

[0045] storing filtrate obtained from filter (18) in separate tank (19) and mixing it in slacker (7) in control way using a magnetic flow meter; and

[0046] feeding high density mud obtained by filter (18) 1300-1500 kg/m3 in a separate mud mixing tank for further washing.

[0047] White liquor is most important alkaline cooking chemical and supplied by chemical recovery process department to pulp mills to solublize all lignin in wood chips/Bamboo/wheat straw/rice straw/flax reed/other fibrous raw material. A typical white liquor is whisky color solution having pH-14, TTA-100-145 g/L, AA-90-128 g/l as Na2O. TTA stands for total titarable alkali (NaOH +Na2S+ Na2CO3). AA stands for (NaOH +Na2S). Green liquor of TTA-100-140 g/l is allowed to react with active lime CaO (more 75% purity) in slacker -1 up 78% causticizing efficiency. For instance let us see properties of incoming green liquor to slacker-1 (before reaction)

TTA-140 g/l as Na2O (Na2CO3-100 g/l, Na2S-25 g/l, NaOH-15 g/l)
AA-40 g/l
Causticizing efficiency-13% NaOH/(NaOH+Na2CO3)
Sulphidity-40%
Unreduced Sodium Sulphate-5 g/l as Na2O
Na2CO3/Na2SO4= 20

a. Na2CO3 (green liquor) + CaO (quick lime) + H2O = 2 NaOH + CaCO3 (lime mud)

[0048] After first stage causticizing reaction white liquor will have following parameters:

TTA-140 g/l as Na2O (NaOH-90 g/l, Na2S-25 g/l, Na2CO3-25 g/l)
AA-115 g/l as Na2O
Causticizing Efficiency-78% NaOH/(NaOH+Na2CO3)
Sulphidity- 21.7% (Na2S/AA)
Unreduced Sodium Sulphate-5 g/l as Na2O
Na2CO3/Na2SO4= 5

[0049] As you see that just 78% of Na2CO3 is converted in to sodium hydroxide or reaction is just 78% complete.

[0050] (The term Causticity means that amount by which Na2CO3 get converted in to NaOH in this invention 95% causticity means 95% of conversion of Na2CO3 to NaOH).

[0051] A typical White liquor is of following parameters {if TTA 100-140 g/l then NaOH 70-86 g/l, Na2S 18-30 g/l, Na2CO3 12-24 g/L, Sulphidity 18-26% (Na2s/(Na2s+NaOH)), Causticity 78-82% (NaOH/(NaOH+Na2S))}. In Industry across globe Caustic zing reaction is kept 78-80% (causticity). Because beyond this free lime starts to increase along with NaOH generated, which clog Filter cloths with these fines and also make the white liquor turbid. Except active alkali (NaOH and Na2S) every other solute present in white liquor is dead load and should be minimized as much possible. Also to be note down that White liquor also contains 3-5 g/L unreduced Na2SO4. Form Inventor point of view Na2CO3 must be reduced as much as possible so that weight ratio of {Na2CO3/(Na2CO3+Na2SO4)} is as low as possible because Na2CO3 above 28% solubility starts to precipitates as crystal in evaporator’s black liquor solids (> 66% black liquor solids) see the drawing no.3. To have this happen Inventor has proposed to increased causticity up to 95% in two steps
[0052] Step 1: To react raw green liquor 100-140 g/L as Na2O with lime up to 78-80% causticity in slacker-1. Filtering out white liquor from 4, 5 (drawing .2 ) so generated and reacting the White liquor of step 1 in step2 as follow

[0053] Step 2: White liquor produced from 4,5 in stage 1 (Drawing.2) is allowed to react with lime form lime bin(17) in slacker (7) till causticity 95%.

[0054] Na2CO3 (Residue of White liquor) + CaO (quick lime) = 2 NaOH + CaCO3 (lime mud)

At this stage-2 White liquor constituents becomes as follows
TTA-140 g/l (NaOH- 109 g/L, Na2S-25 g/L , Na2CO3-6 g/L)
AA-134 g/l as Na2O (NaOH- 109 g/L+ Na2S-25 g/L )
% Sulphidity -22% ,
% CE- 95%
Unreduced Sodium Sulphate-5 g/l as Na2O
Na2CO3/(Na2SO4)= 1.2

[0055] Slurry hence formed is transferred to WLC-2 (10) and its overflow is stored in WL feed tank (21) then transferred to WL service tank (24) through a cylindrical mesh filter (23). Unreacted lime mud of WLC2 (10) is continuously withdrawn and fed to Slacker (1) via 11,12,13,14. Here unreacted lime mud is fully neutralized. Till now one can see that Na2CO3/(Na2SO4) = 1.20. Before 95% Causticity it was Na2CO3/(Na2SO4) = 5 and in the raw green liquor this was 20. So it is clearly evident from the drawing .3 Evaporator Scale will only be in the range of disulphate (burkeite). As we all know that Burkeite can be easily washed away with hot water/weak white liquor/Weak black liquor boiling. This is what inventor wanted to.

Part-B: This step will done in pulp mill site
[0056] In Kraft MME hard scale like alumina silicate or calcium silicate totally because of raw material and type of raw material used. Also be note down that one of the major and notorious most inorganic material is NaCl and KCL. Albeit these does not exhibit scales on evaporator heat transfer areas but these causes passage jamming of recovery boiler superheater area and others. NaCl and KCl content are found to be highest in agro based raw material like Wheat straw and Rice husk and other wood material that belong costal area. And these must be removed in wood washing stage itself otherwise. These are going to accumulate and will remain in the chemical recovery cycle. Inventor has found that washing raw material with evaporator secondary condensate (at temp 90 deg C or above) is really effective in removing NaCl and KCL along with silica hence by purging the wash water intermittently.

[0057] Inventor has also found that 50% calcium attached (500g -1000g/ton wood)with raw material can be removed by washing the raw material obtained above with diluted soda ash (Na2CO3) as per reaction below.

[0058] Na2CO3 + CaO & Ca++ ions (raw material calcium) +H2O CaCO3 +2 NaOH

[0059] Which can be removed by a conical tank system as raw material being lighter in density floats above and classified simply. To a greater extent silica also removed in this process.

[0060] Now the turn is of alumina removal. Inventor also found that NaOH so formed reacts with alumina in raw material to form Sod .meta –aluminate followed by aluminum hydroxide and is removed as per reaction follows:

1. Al2O3+2NaOH 2NaAlO2 +H2O
After dilution with water, stirring and skimming off
2. NaAlO2 + 2H2O Al(OH)3+2NaOH
3. 2Al(OH)3 Al2O3+3H2O

[0061] As aluminum content are in PPM in raw material , dissolved alumina in the solution can be directly purged off in an acidic media or send back recovery process ‘s Dregs washers or Regenerated as Na2CO3.

[0062] As in mill Green liquor or dregs wash content major amount of Na2CO3 so this can be supplied to chips washing system.

[0063] Inventor has also find out that solution obtained by reaction can be allowed to react with CO2 gas of Rotary lime kiln flue gas which is easily available at every paper mill and Na2CO3 can be again regenerated as follows

4. 2NaAlO2 +CO2+H2O 2Al (OH) 3+Na2CO3 and can be skimmed off again
[0064] Soda ash solution so obtained is sent back to raw material washing and the process remains cyclic.

Advantages of the Present Invention

[0065] More white liquor Generation give an edge to mill increase pulping production, Which means more Black liquor solids and more steam generation, which in turn leads Lower Cost of production which in turn leads to lower cost of pulp.

[0066] Calcium and silica removal helps less scaling on heating surface which in turn lower viscosity in Kraft black liquor even at higher solids.

[0067] Lower Calcium and silica in WBL leads to lesser scale on heat transfer area in MEEs
[0068] Transition from dicarbonate (2Mol Na2Co3 and 1 Mol Na2SO4) to disulphate(Burkite: 2Mol Na2SO4 and 1 Mol Na2CO3 ), Gives easy scale which can be removed just by Hot water / Weak Black Liquor/ Weak White liquor Boiling.

[0069] High pressure water jetting cost reduces substantially also down time of of MEEs

[0070] Removal of Silica and Calcium helps in Kraft recovery boiler smelt handling, jamming tendency reduces greatly.

[0071] Reduction in dead loads leads lower boiling point elevation in MEEs and performance improvement of MEEs

[0072] Due to reductions of inorganic compounds in Weak black liquor and increased organic compounds more steam is generated in Recovery furnace.

[0073] As scale transition moves from dicarbonate to disulphte there is a further chance to increase black liquor solids concentration up 85% (dryness) easily and up to 90% with bit modification in Multiple effect evaporation plant.

[0074] While the disclosure has been presented with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit and scope of the disclosure. It is intended, therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the disclosure.