|Title of Invention||
A PROCESS FOR THE DECOLOURISATION OF CELLULOSE PULP-MILL WASTE WATERS,
|Abstract||1.A process for the decolourisation of cellulose pulp-mill waste waters comprising the steps of treating said waters with at least one reagent selected from compounds of calcium, iron, titanium and susbstances containing the same; thoroughly agitating the reaction mixture; separating therefrom, by filtration, the resulting solid phase or precipitate containing ligneous colour bodies; precipitating soluble calcium salts along with futher ligneous colour bodies, if any, from the filtrate, by carbonation.|
This invention relates to a process for the decolourisation of cellulose pulp-mill waste waters with combined chemical recovery.
in the cellulose pulp manufocturing industry, using wood and reeds as raw materials, very dark coloured effluents of substantial quantities are generated. It is necessary to decolourise these waste waters prior to discharge into &e environment as stipulated by pollution control authorities.
Different mills use different different methods of pulping namely acidic or neutral prehydrolysis followed by sulphate or sulphite cooking. Waste waters of different chemical conpositions emerge from these different operations. The pehydrolysis liquor and wadi after removing the cooked cellulosic materials is termed the prehydrolysis waste water which is discharged to the effluent treatment plant without chemical recovery. The sulphate or sulphite cooked liquors are collected and sent for recovery of the chemical values therein contained. At this stage &e pulp is washed with water and the dark coloured wash water is too dilute for economical recovery of the chemical values and as such is discarded as effluent This coloured liquor is usually termed as the pulp wash.
The prehydrolysis waste water and the pulp wash water are different in chemical conpositions.The prehydrolysis waste water is slighty acidic m nature while the sulphate process pulp wash water is alkaline in nature and the sulphite process pulp wash water is acidic in nahire.
Conventional methods of treatment of these waste waters as practised by &e industry include neutralisation with lime, anaerobic bio-degradation followed by aeration and clariiicalion by sedimentation or filtratioa However, these operations are not successful in removing &e colour of the waste waters to the desired level.
This invention proposes a cost effective process for ttie decolourisation of pulp mill coloured ( sepanrtely or in combination by utilising the byproducts available
jEhom the industry itself
Waste water samples collected from two different pulp mills were treated in accordance
Hi? wash water thus generated is &e pulp wash waste liquor containing tiie extracted ligneous colour bothes and is alkaline in nature. The cook liquor prior to pulp washing stage is concentrated by ev^oration and burnt in a suitable furnace. TTie ash generated by this burning conteuns sodium carbonate v^ich is dissolved in water and treated widi calcium hydroxide to precipitate calcium carbonate and to convert the sodium values back to sodium hydroxide which is returned to the process cycle. Hie calcium carbonate precipitate generated at this stage of operation is burnt in a suitable kiln to recover the calcium values as calcium oxide is recycled into the process cycle for finlfaer
caustisisstioa Hiis calcium carbonate precipitate is hereinafter also referred to as lime sludge.
recycled into the process and flie kiln gas may be JB-eed from suspended matter and used to advantage, since it will normally contain from 20 to 45% carbon dioxide. Hie treated aqueous portion of the effluent now would have attained a colour reduction of 80 to 95%. The pH of flie treated effluent would be in the hig|her level pH of 9.5 to 11.7. ITiis may be &rtfter treated with carbon dioxide containing gases to reduce the pH to neutral levels of 6.5 to 7.5 and preferably to 7.0 for discharge. In plants where the prebydrolysis is practised prior to pulping the prefaydrolysate liquor contain&colour bothes different from that present in Ae pulp wash water essentially flavones and antfarocyanines and &e above, treatment affords only 70 to 90% of colour removal for tiiis effluent Quantify wise the ratio of prefaydrolysis waste water to pulp wash water effluents in a typical mill is in the range of 1:30 to 50 and as such the prefaydrolysis waste water shall be provided with the following treatment for substantial colour removal in the range of 80 - 99% as desired. The following methods can be adopted either alone or in combination as desired either prior to or after the calcium salt treatment as described earlier.
Addition of compounds of iron either alone or in combination with compounds of titanium and / or calcium either separately or together in quantities varying from 0.02 to 20 gm per litre to the waste effluents containing organic coulour bothes depending on their colour intensity, resists in the conjoint precipitation adsorption of the iron and the organic colour bothes above 2.5 pH. Hiis affords a mediod for decolourisation of the coloured effluents from cellulose pulp mills, the dark coloured spent wash from distilleries using molasses and such other effluents containiug dissolved organic colouring materials, lliis mediod could be adopted either alone, prior to or subsequent to the calcium salt carbon dioxide treatment mentioned above and elaborated in Example I
Precipitated colour bothes as detailed above could be flocculated further prior to solid liquid separation using known flocculants and further supplemented by the use of filter aids known to (he industry.
The wash water from thepulp bleaching plant containing un- utilised active bleaching agents such as chlorine, hypochlorite, chlorine dioxide etc. can be effectively blended with &e prelordrolysis waste water initially treated by the calcium salt method to decolourise the efQuent to above 95% of the initial colour. This in no way limits the use of firesh bleaching agents like chlorine gas, chlorine water, hypochlorite or chlorine dioxide in theplace of the
wash water from the bleaching plant
the process for the decolourisation of cellulose pulp-mill waste waters, according to this invention, comprises the steps of treating said waters with at least one reagent selected from compounds of calcium, iron, titanium and susbstances contaning the same; thoroughtly agitating the reaction mixture; separating therefrom, by filtration, the resulting solid phase or precipiMe containing ligneous colour bothes; precipitating soluble calcium salts along witih frnrther ligneous colour bothes, if any, from the filtrate, by carbonation.
Ja atypical example, a sample effluent was collected from a sulphate route wood pulping plant and it measured 1,500 to 2,000 true colour units as deteimined by the known platinum-cobalt method of detennining colour values.
A quantity of raw affluent was mixed with a quantity of CaO as lime slurry while at the same time another quantity of raw effluent was thoroughtly mixed witii a known quantity of lime sludge (essentially CaCO3from the caustisization plant as detailed before). The two resultant slurries were intimately mixed in such a way that the mixture now contained in the range of 200 to 250 parts per million of calcium hydroxide calculjtted as Ca(0H)2 and 250 to 400 parts per million of lime sludge calculated as CaCO3. The pH of the mixture nowij^easured in the range of 11.0 to 13.0 v^ich is required to affl)rd (he precipitation of the colour bothes. The mixture was kept in agitation for 15 minutes to 3 hours prefers^ly for 1 hour and the solid phase was separated by filtration. Hie filtrate was treated with CO2 gas to a pH of 11-12 \^dlen precipitation of the dissolved calcium salts as calcium carbonate and conjoint agglomeration separation of the residual colour bothes occur Bimoltaneously. The second solid phase thus formed was filtered and removed. Tlie filtrate had a reduction of 95% of its original colour units at this point. Tlie filtrate was fiirOier treated witii CO2 gas to reduce the pH to 7.0. Higher concentrations of hydrated lime (calculated as Ca(0H)2 and lime sludge (calculated as CaC03) will be required for ligneous waste liquors hsviqg higher colour units (calculated by the platimim-cobalt metfiod described earlier).
Generally thee raw waste effluent hydrated lime and lime sludge are combined in such relative proportion and quantities fliat CaCO3provided by the lime sludge is present in the mixed slurry in an amount which is at least equal to the hydrated lime provided by Ca(0H)2. Use of the lime sludge (essentially CaCOj) desirably and signification reduces fte amount of l^drated lime (Ca(0H)2 that migjit otherwise be required to counteract the level of the colour bothes in the same waste effluent to be treated.
Hie combined calcium carbonate and colour bodies comprised in the two solid precipitates from the two stages of treatment is conveniently burnt in a lime kiln to bum the organic materials and to convert and recover calcium values therein as CaO.
A sample of 10 litres of mixed effluent was collected from a pulp mill practising prebydrolysis prior to sulphate cooking. The sample effluent contained the prehydrolysis waste water and flie pulp wash waste liquor in flie ratio of 1 : 32.5 by volume ^nd measured 1700 colour units as determined by the platinum cobalt method. Hiis imtroued effluent was taken in a vessel fitted with a sturer coimected through a stepless speed regulator.
Ihe stirrer was switched on and run at 100 rpm. 10 millilitreB of a prepared solution containii^ ferric sulphate and titaiiyl sulphate were added in drops while keeping the solution agitated.
Ihe ferric sulphate titanyl sulphate solution was prepacd by oxidising with hydrogen peroxide die effluent obtained from a sulphate route titanium dioxide plant following the ilmenite sulphuric acid process and analysed: TiO SO - 12 gm per litre, Ferrous Sulphate -180 gm per litre and free sulphuric acid - 200 gm per litre. One litre of tfiis solution was reacted with 0.6 millilitres of a 10 % hydrogen peroxide solution to oxidise all the ferrous iron in solution to ferric iroa
1.A process for the decolourisation of cellulose pulp-mill waste waters comprising the steps of treating said waters with at least one reagent selected from compounds of calcium, iron, titanium and susbstances containing the same; thoroughly agitating the reaction mixture; separating therefrom, by filtration, the resulting solid phase or precipitate containing ligneous colour bodies; precipitating soluble calcium salts along with futher ligneous colour bodies, if any, from the filtrate, by carbonation.
2. A process as claimed in Claim 1 wherein the said waste waters are treated with a reagent containing ferric sulphate and titanyl sulphate.
|Indian Patent Application Number||1618/MAS/1996|
|PG Journal Number||30/2009|
|Date of Filing||17-Sep-1996|
|Name of Patentee||MOHAN VISWANATHAN NAYAR,|
|Applicant Address||68 ADARSH NAGAR, TRIVANDRUM 695 004|
|PCT International Classification Number||D21C9/10|
|PCT International Application Number||N/A|
|PCT International Filing date|