|Title of Invention||
"A PROCESS FOR THE RECOVERY OF SODIUM SULFITE FROM WEAK BLACK LIQUORS AND THE APPARATUS THEREOF"
|Abstract||The present invention relates to a process and the apparatus for the recovery of sodium sulfite from weak black liquor obtained from agricultural residue pulping.which comprises concentrating by evaporation weak black liquor in multi-effect short tube evaporator to solids content, mixing said concentrated weak black liquor with agricultural biomass obtaining thereby a dark colored semisolid mass, burning said semisolid mass to form ash, leaching said ash in water and filtering same to obtain solution of sodium silicate and sodium carbonate, heating the said solution and passing SO2 to complete precipitation of silica and converting all sodium into sodium sulfite/ bisulfite, separating the said precipitated silica from sodium carbonate solution , raising the pH of the filtrate to above 7.5 by addition of caustic soda to convert bisulfite into sulfite.|
|Full Text||FIELD OF THE INVENTION
The present invention relates to the process for recovery of sodium sulfite from weak black liquor and apparatus thereof and more particularly from black liquors obtained from cooking of agricultural residues such as rice straw, wheat straw, sarkanda, husks and the like with sodium sulfite.
In the known art so far as paper making is concerned, cellulosic raw materials like wood, bamboo, agricultural residues, are cooked with chemicals, principally caustic soda/ sodium sulfite to separate and obtain cellulosic fibers ( known as pulp) for further processing into paper. The other substances from the above mentioned raw materials, mainly lignin, combine with caustic soda/sodium sulfite for removal from the pulp solution with water when the pulp is washed for the above mentioned separation. This solution is known as black liquor principally due to its dark blackish colour. The black liquor also contains other substances like pentosans, sugars, fiber fines, silica and other non performing chemicals like salts of magnesium and calcium (obtained from the raw material), and the like, which make up total solids in the black liquor. However, in mills where wood or bamboo is used as raw material, the black liquor obtained as above is concentrated to 60% solids content and burnt in a furnace. The residual ash is obtained as soda ash and is converted into caustic soda by causticizing it with lime. This process is possible because the black liquor contains little or no silica and other non performing chemicals and it can be concentrated to a solid content of 60% or more, at which concentration it can burn on its own.
In mills where agricultural residue is used as raw material for paper, the process mentioned above for recovery of sodium sulfite from black liquor can not be used for the following reasons :
The raw materials have very high silica content due to which the ash obtained after burning of concentrated black liquor( soda ash) will contain sodium silicate in substantial quantities.
The black liquor can not be concentrated beyond 45% solids content due to presence of excessive amounts of fiber fines. Concentrated black liquor at this low concentration can not be burned on its own and needs external fuel( like fuel oil) support to burn, which, besides posing process difficulties, becomes a very expensive proposition. Burning of the concentrated black liquor is only known way for recovery of sodium sulfite from it.
Since the black liquor cannot be burned, there is no alternative for the mills using agricultural residues but to discharge the weak black liquor as effluent. This discharge causes tremendous amount of pollution and is an environmental hazard.
Various attempts have been made to overcome these disadvantages.The most prominent and extensively used procedure has been to desilicate the black liquor. The said process has some basic flaws due to which it has not been successfully implemented till date.
Firstly ,in the process of desilication, it is not exactly known when all silica has been precipitated. Either some silica remains behind which interferes with the subsequent recovery process or, due to excessive carbonation besides silica, some lignin is also precipitated which lowers the heat value of concentrated black liquor and subsequent combustion even at 60% solids content concentration without external fuel support becomes difficult.
It is very difficult to maintain an exact pH level of 9.5 which is an essential requirement of the process. Even slight variation from this pH causes lignin precipitation resulting in lowering of the heat value of the concentrated black liquor.
Even after silica precipitation, it is difficult to concentrate black liquor up to 60% solids content since the fiber fines are not removed, causing problems in the evaporator system due to choking.
The precipitated silica also is a disposal problem and its disposal is an environmental hazard.
Indian patent number 171289 dated 18th December, 1987 describes a process of recovery of caustic soda in which some of the the aforesaid disadvantages are overcome. However the following factors have not been taken into consideration by the said patent:-
A. The combustion of the concentrated black liquor- bio mass mixture can be
carried out at higher temperatures in the range of 820 °C- 950 °C instead of
earlier range of 750 °C - 820 °C. This elevated temperature results in better and
complete combustion and a higher sodium silicate to sodium carbonate ratio
leading to higher yields.
B. Like in case of other black liquors, multi effect tubular evaporators can be
used to concentrate the weak black liquor except that the tubes have to be of
shorter length to avoid in tube boiling and at any time this black liquor can not be
subjected to a temperature greater than 110 °C. Besides silica and fiber fines
there are non performing chemicals like salts of calcium and magnesium
contributed from raw materials which do not interfere in the described evaporation process.
C. If a counter current leaching of at least three stages is used to dissolve sodium silicate/ sodium carbonate from the ash, a much higher concentration of sodium in terms of Na2O (10 - 12 % as against originally proposed 3 - 5%) resulting in elimination of subsequent concentration step of the sodium carbonate solution.
The present invention relates to a process of producing sodium sulfite from sulfite pulp recovered from black liquors obtained from cooking of agricultural residues such as, rice straw, wheat straw, sarkanda, husks and the like with sodium sulfite.
An advantage of the present invention is that it eliminates the need for concentrating the black liquor of 45% solids content to a higher solids content at any time during the process.
Another advantage is to use/mix cheap and abundantly available agricultural bio mass as additional fuel with the low concentrated black liquor for augmenting the heat value of the black liquor.
A further advantage of the invention is the ease with which the aforesaid mixture of the agricultural bio mass and black liquor( hereinafter referred to as wet mixed fuel or WMF) can be handled because of its semisolid state.
The object of the present invention is therefore a process for the recovery of sodium sulfite from black liquor.
Another object of the invention is to provide a process, which eliminates the need for concentration, the black liquor of 45% solids content to higher solids content.
A further object of the invention is to provide cheap and abundantly available agricultural bio mass as additional fuel.
Yet another object of the invention is to provide rice husk/ cut wheat straw/ cut rice straw/ saw dust etc., as additional fuel, which is also cheap and abundantly available.
According to the present invention there is provide a process for the recovery of sodium sulfite from weak black liquor obtained from agricultural residue pulping, which comprises concentrating by evaporation weak black liquor to solids content of 40 - 48%, mixing said concentrated weak black liquor with agricultural biomass in the ratio of 2:0.8 to 2:1.5 by weight obtaining thereby a dark colored semisolid mass (WMF), burning said semisolid mass to temperature of from 820°C - 950°C to obtain ash, leaching said ash in water and filtering same to obtain solution of sodium silicate and sodium carbonate, heating the said solution to above 90°C, passing SOi gas obtained burning of sulfur, reducing the pH to below 6.0 to complete precipitation of silica and converting all sodium into sodium sulfite/ bisulfite, separating the said precipitated silica from sodium carbonate solution, raising the pH of the filtrate to above 7.5 by addition of caustic soda to convert bisulfite into sulfite.
Preferably, the agricultural bio mass include cut rice straw, cut wheat straw, and the like. Of these, the most preferably is rice husk. Another important and
abundantly available waste would be saw dust. Saw dust either individually or in combination with rice husk in any proportion can be used.
The present invention can better be understood with the reference to the accompanying drawings which are for illustrative purposes and should not in any way be construed to restrict the scope of the invention keeping in view that certain modifications and improvements are possible without deviating from the scope of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 Schematic diagram of the process DETAILED DESCRIPTION OF THE INVENTION
According to the process, the weak black liquor obtained from the washing of the pulp is concentrated by evaporation in short tube multi effect evaporators having the tube lenghth between 1.8m-3m such that at no point during the evaporation process, the temperature of black liquor exceeds 110° C to solids content of 40 -48%, preferably 45%. This concentrated black liquor is mixed with agricultural bio mass in the ratio varying from 2:0.8 to 2:1.5, preferably 2:1 on weight to weight basis. After thorough mixing, a dark coloured semisolid mass is obtained, known as WMF. This WMF is then burnt in a suitable preferably travelling grade furnace so that the temperature of oxidation remains at around 820°C and, in any case, does not exceed 950°C. As soon as the combustion is complete, the resultant ash is immediately removed from the furnace. It is important that the ash should not be left in the hot furnace after combustion. The ash thus obtained contains a mixture of soluble sodium silicate and sodium carbonate in the ratio varying from 75:25 to 80:20, preferably 78:22. The resultant ash is atleast three
stage counter currently leached with water and the mixture filtered through a suitable medium, or settled to remove insoluble portion of the ash and recover the sodium silicate/sodium carbonate solution. The solution thus obtained contains sodium measured as Na2O as 10-12 % of the solution.
The above solution is heated to 90°C and sulfur dioxide gas passed through solution under agitation.The sulfur dioxide gas is generated by burning of sulfur by known means. Samples are drawn from time to time to measure the pH. When a pH value of 5.5 to 6.0 , preferably 6.0 is reached , the sulfitation reaction is complete. The sulfur dioxide gas reacts with sodium silicate to form sodium sulfite/ bisulfite and silica is precipitated. Sulfur dioxide gas also reacts with the sodium carbonate present in the solution to form sodium sulfite / bisulfite and carbon dioxide gas which gets blown off. At pH of 6.0 all silica is precipitated and settles and is removed from the sodium sulfite/ bisulfite solution , preferably by filtration. The recovered silica is washed with water and hydrochloric acid and dried. The pH of sodium sulfite/ bisulfite solution is raised to 9.0 by addition of caustic soda to convert all bisulfite into sulfite. The solution thus obtained contains 12 - 15 % of sodium sulfite. This process effects a recovery of sodium sulfite ( measured as Na2O) as a percentage of the sodium obtained from original black liquor to the tune of 85 to 88 % , preferably 87 %.
The following examples of various stages of operation are based on pilot plant operation. The data for example are presented on stage wise operations, maintaining a percentage wise link with the sodium content ( measured as Na2O) , where ever applicable. The sulfitation results are based on laboratory experiments.
For evaporation five effect short tube evaporators were used, each having multiplicity of tubes of 0.035 to 0.045 meter inner diameter and 2.5 meter effective length. The steam for this operation was supplied from the main paper plant initially and after wards from the recovery boiler at 1.5 kg/cm2 pressure. For the measured run, a weak mixed black liquor was obtained from the pulp mill from the pulping of rice straw, wheat straw and sarkanda. The solids content in the weak black liquor were around 5% and Na2O content about 1.17%. After evaporation 9.3 m3 of concentrated black liquor was obtained having a solids content of 41.9% and Na2O content of 6.15 % or equivalent to 0.572MT , which is 97.77% of the original. The measured run was operated for 24-28 hours .
Mixing and Burning
5 m3 of concentrated black liquor obtained after evaporation were thoroughly mixed with air dry rice husk in continuous single screw mixer of with stub agitators to obtain Wet Mixed Fuel containing Na2O. This slight increase in Na2O was attributed to Na2O present in rice husk.
The Wet Mixed Fuel was fed on to a travelling grate furnace. High speed Diesel was burnt through four burners under neath the beginning of the grate travel to heat the fuel to its ignition temperature of 160-190°C. There after air was supplied from underneath the grate to create and carry to end the combustion. A probe was installed approximately mid way of grate travel to note the high temperature of combustion . At the end of the travel, the ash was dropped into a pit to be collected for subsequent operations. The collected ash was weighed and its Na2O content determined. Its weight was found to be 1.11 MT having a Na2O content of 27% or 0.299MT. This corresponds to 97.4 % of Na2O from
concentrated black liquor or 95.2 % of original from pulp mill. The sodium silicate to sodium carbonate ratio was found to be 78: 22, the unburnts were determined ( after complete incineration in muffle furnace) to be 4.9 %. The total time taken for the run was 12 to 13 hrs including mixing and furnace preheating time.
For leaching, 1050 kg of ash obtained after burning was leached in three stage counter currently. Each stage is provided with an open barrel, with a closed steam coil for heating, a bottom valve for extraction of leached solution and a cage made up with bottom and sides of almost 15-25 mesh screen whose diameter and height were 60% of that of the drum , with a handle extending at the top to lift the cage up and down for shaking purposes. The cage was such that it would handle 50 kgs of ash at a time to be extracted with initial 100 liters of fresh water. The three barrels were arranged in a triangle, so that each barrel could become stage one. The leaching sequence for initiation was conducted in 14-16 steps and subsequent cyclic operations in 25-27 steps per cycle. After leaching and filtration to remove suspended insolubles , the filtrate obtained was having 95-96% of Na2O from ash or 90-92% of Na2O from pulp mill. The total filtrate contained Na2O, silica, other solubles and water
1 kg of leached solution obtained from leaching trialwas taken in a 1.5 liter sintered funnel fitted on top of a buchner flask heated to 90°C using immersion heater and sulfur dioxide gas ( commercially obtained in cylinder) bubbled through from under the funnel. A continuous pH probe was inserted into the solution to monitor the lowering of pH due to reaction of SO2 gas with sodium silicate and sodium carbonate , converting them both into sodium sulfite /
bisulfite . In approximately 15 minutes , the pH reached 5.5 and the total silica was in a precipitated/ suspended state.The SO2 gas was stopped and the contents filtered through buchner funnel with application of vacuum till the precipitate was substantially dry. The liquor collected measured 995 gms and Na20 content of 10.7% or 98.6% of leached solution or 89.6% of Na2O from pulp mill.
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
1. A process for the recovery of sodium sulfite from weak black liquor obtained
from agricultural residue pulping, which comprises concentrating by evaporation
weak black liquor to solids content of 40 - 48%, mixing said concentrated weak
black liquor with agricultural biomass in the ratio of 2:0.8 to 2:1.5 by weight
obtaining thereby a dark colored semisolid mass (WMF), burning said semisolid mass
to temperature of from 820°C - 950°C to obtain ash, leaching said ash in water and
filtering same to obtain solution of sodium silicate and sodium carbonate, heating the
said solution to above 90°C, passing SO2 gas obtained burning of sulfur, reducing the
pH to below 6.0 to complete precipitation of silica and converting all sodium into
sodium sulfite/ bisulfite, separating the said precipitated silica from sodium carbonate
solution, raising the pH of the filtrate to above 7.5 by addition of caustic soda to
convert bisulfite into sulfite.
2. The process as claimed in claim 1 wherein the said weak black liquor is
concentrated to a solids content of 45%.
3. The process as claimed in claim 1 wherein the said agricultural biomass are
rice straw, wheat straw, sarkanda, husk, saw dust and the like.
4. The process as claimed in claim 3 wherein the said husk is the rice husk.
5. The process as claimed in any of the preceding claims wherein the ratio of the
concentrated black liquor to agricultural bio mass is 2:1 by weight.
6. The process as claimed in any of the preceding claims wherein the burning of
the semisolid mass is effected at temperature of 860 - 880°C resulting in ash, being a
mixture of soluble sodium silicate and sodium carbonate.
7. The process as claimed in any of the preceding claims wherein the ash is
leached with water to obtain solution of sodium silicate and sodium carbonate,
containing sodium measured as Na2O from 10 to 12% of the solution.
8. The process as claimed in any of the preceding claims wherein, pH value of
said solution of sodium silicate and sodium carbonate is reduced to pH value between
5-6, preferably 6.
9. The process as claimed in claim wherein the said silica is washed with water
and HC1 and dried to make precipitated silica as a by product.
10. An apparatus for the recovery of sodium sulfite from weak black liquor as
claimed in claim 1 comprising:
a pulp mill for making a pulp from the raw material and chemicals,
- a multi-effect short tube evaporator, preferably five effect short tube
evaporator to condensate weak black liquor obtained from the pulp,
- a mixer for mixing the black liquor and agricultural biomass to obtain wet
a furnace for burning said wet mixed fuel,
a three stage leacher for leaching ash,
conventional means for sulfitation and alkalization to recover sulfite.
11. An apparatus for the recovery of sodium sulfite from weak black liquor as
claimed in claim 10 wherein length of the tube of said multi-effect short tube
evaporator is between 1.8m to 3m.
12. An apparatus for the recovery of sodium sulfite from weak black liquor as
claimed in claim 10 wherein the said furnace for burning the wet mixed fuel is a travel
13. A process for the recovery of sodium sulfite from weak black liquor
substantially as herein before described with reference to the foregoing example.
|Indian Patent Application Number||619/DEL/2002|
|PG Journal Number||13/2009|
|Date of Filing||07-Jun-2002|
|Name of Patentee||MAHESH KUMAR KHAITAN|
|Applicant Address||28, SECTOR 9A, CHANDIGARH.|
|PCT International Classification Number||D06B 23/00|
|PCT International Application Number||N/A|
|PCT International Filing date|