A pulping method using KOH-K2SO3-AQ system as cooking liquor on wheat straw was studied in this paper. Digestion quantities on pulp yield and hardness during cooking was discussed and determined. It looks promising to use effluents of cooking into fertilizer because it contains rich nutrition such as potassium and lignin. A new pattern of ecological cycling may be set up between paper industry and farming

Jiang Qi-pei

Li Zuo-hu

Mo Hai-tao

Zhang Xiao-yong

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CleanerProductionofWheatstrawPulpwithPotashJiang Qi-pei, Zhang Xiao-yong,* Mo Hai-tao, and Li Zuo-huState key Lab of Biochemical Engineering, Institute of Process Engineering,Chinese Academy of Sciences, Beijing 100080, P.R.ChinaA pulping method using KOH-K2SO3-AQ system as cooking liquor on wheat strawwas studied in this paper. Digestion quantities on pulp yield and hardness during cookingwas discussed and determined. It looks promising to use effluents of cooking into fertil-izer because it contains rich nutrition such as potassium and lignin. A new pattern of eco-logical cycling may be set up between paper industry and farming.Key words:KOH-K2SO3-AQ system; cooking; wheat straw; ecological cyclingIntroductionCrop straw is one of the most abundant renew-able resources in the world, but paper made fromcrop straw contributes to less than 8 % of global pa-per production.3 The main reason is that black li-quor from straw digestion has a high content of sili-cate and high viscosity. This leads to low alkali re-covery ratio and high costs to black liquor disposal.Effluents from paper industry have become thelargest contaminants on environment in China.However, the pulp output is insufficient to meet theincreasing demand for paper, particularly in the de-veloping countries. This results in an increasingshortage of wood raw materials and gradual defor-estation of some areas in China. Thereforenon-wood materials such as wheat straw and vari-ous other agricultural residues are so abundant andespecially attractive for pulp-making. Nowadays,paper made from crop straw still contribute 60 % tototal output in Chinese paper industry despite thefact that the Chinese government had cracked downlarge number of paper mills due to pollution fromblack liquor. Pollution from black liquor has beenthe most serious factor which hinders developmentof pulp and paper industry in China.10Potassium-base chemical pulp of agriculturalresidues offers a novel approach to dispose of spentpulp liquor in a safe and economical way. Foelkeland Milanez6 studied the use of chemicals wheresodium was substituted by potassium in kraft andsoda pulping of Eucalyptus. Their test result indi-cated that a satisfactory pulp could be obtained withK-based pulping chemicals. H’ng4 reported thatgood pulp yield and low kappa number wereachieved in potassium hydroxide-anthraquinone(KOH-AQ) pulping of oil palm fibers comparableto that of soda pulping. The pulp has a better beata-bility and brightness than the corresponding pulp-ing from soda cooking. Recently, Nolan2 reportedthat K-base has more efficient delignification abil-ity than conventional Na-base pulping. Subse-quently, Arbokem Inc2 (Canada, 1991) proposed apulping system using K2SO3, in which the spent li-quor can be recovered into valuable K-fertilizer. Insome developing countries such as China, straw-pulpstill domainates the market of paper industry.K-base pulping could be a new approach to controlpollution from black liquor.A new pulping technique was studied in this ar-ticle that using KOH-K2SO3-AQ system as cookingliquor to digest wheat straw. The purpose of thispulping system is to eliminate pollution by black li-quor from straw-pulp without changing the mainpulping technique during pulping production inChina. The important advantage of this cookingsystem lies in its effective recovery system.Effluents of this cooking system could be trans-formed into inorganic or organic fertilizer by meansof alkali recovery apparatus and cleaner productionmight be set up in the straw-pulp mills.Material and methods2.1 Wheat straw was obtained from the suburbof Beijing. Its composition was determined accord-ing to reference 5 and is listed below: ash content7.69 %, nitric acid/ethanol cellulose 41.25 %,klason lignin 16.38 %, acid-soluble lignin 2.29 %,total lignin 18.67 %, cold water-solubles 11.82 %,hot water-solubles 17.63 %, 1 % NaOH-solubles42.53 %, ethanol/benzene extractable 2.74 %, andpoly-pentose 22.94 %.2.2 Pulping experiments were carried out in15 L stainless steel autoclave rotated digester whichJ. QI-PEI et al., Cleaner Production of Wheat straw Pulp with Potash, Chem. Biochem. Eng. Q. 20 (1) 107–110 (2006) 107*Corresponding author. Tel.: +86-010-62568173; Fax: +86-010-82623436.Email address: cellulose420@163.com or jqpcom410@sohu.comOriginal scientific paperReceived: February 28, 2005Accepted: December 10, 2005contains four 1 L reactors. Wheat straw was cut to2–3 cm long pieces and was presoaked in the cook-ing liquor for 2 h, and then fed into the reactors.The temperature was increased from room tempera-ture to the preset maximum temperature and thenkept constant for a given period of time. Aftercooking, the black liquor was separated from thepulp by filtration (100 mesh) and the pulp was thor-oughly washed with water. Then the pulp and blackliquor were analyzed respectively. Kappa number,pulp yield, burst index, and tear index were deter-mined according to the methods in reference 5.Results and discussionsEffect of total alkali charge and alkali massfraction on pulp yield and hardnessTotal alkali charge is the total amount of KOHand K2SO3 (expressed as Na2O to 100 g oven-drywheat straw ). Alkali mass fraction is KOH ratio,namely,mm mKOH Na OKOH Na O K SO Na O( )( ) ( ).22 2 3 2This cooking system is mainly alkaline pulpingcooperated with sulfite pulping. The dosage ofKOH is the key factor in the course of deligni-fication. From Fig. 1, kappa number and pulp yieldfalls sharply with the increase total alkali charge.This indicates that high alkali charge can effec-tively accelerate delignification and cellulosedepolymerization, which results in the decrease ofpulp hardness and yield. A satisfactory pulp wasobtained when total alkali charge was 11 % and al-kali mass fraction was 0.6, whose pulp yield is 58.8 %and kappa number is 18.8. It can be determined 11 %(Na2O) of alkali charge and 0.6 of alkali mass frac-tion was the better dosage in this experiment.Effect of digestion temperature and digestiontime on pulp yield and hardnessThe determination of the maximum cookingtemperature (TMAX) and the cooking time at maxi-mum temperature (TC) should be considered com-prehensively. In this experiment, we adopted60min7 the time to maximum temperature to deter-mine TMAX and TC. It was observed from Fig. 2 thatTMAX and TC had significant effect on pulp yieldand hardness. Carbohydrate degradation which oc-curred at highest TMAX and longest TC greatlydropped pulp yield whereas variations of kappanumber was not notable. According to Fig. 2 can beseen, in order to obtain satisfactory pulp yield, 160°C (pulp yield 58.8 %) should be optimal maximumtemperature and 100 min (pulp yield 58.8 %) shouldbe the best time at maximum temperature from ana-lyzing Fig. 2 under the condition of 11 % of totalalkali charge(Na2O) and 0.6 of alkali mass fraction.Effect of AQ and solid-to-liquid ratioon pulp yield and hardnessMass ratio (solid-to-liquid ratio) influences al-kalinity of cooking liquid and homogeneous mixingof wheat straw. From Fig. 3, pulp yield and hard-108 J. QI-PEI et al., Cleaner Production of Wheat straw Pulp with Potash, Chem. Biochem. Eng. Q. 20 (1) 107–110 (2006)F i g . 1 – Effects of the alkali charge and fraction on pulpyield and hardness during cooking (pulp yield: unscreenedpulp yield; pulp hardness: expressed as kappa number)F i g . 2 – Effects of the digestion temperature and time onpulp yield and hardness during cooking (pulp yield: unscreenedpulp yield; pulp hardness: expressed as kappa number)ness decrease with the increase of mass ratio. 1:10mass ratio increases pulp hardness a little becauseof decreasing alkalinity of cooking liquor. 1:7should be the best mass ratio. Additional reagentAQ at low fraction (0.05 %) can effectively de-crease pulp hardness and increase pulp yield (Fig.3). But with increasing concentration of AQ, the in-crease of pulp yield and decrease kappa bumberwasn’t notable. In the experiment, 0.2 % of AQcharge is optimal dosage.Optimal techniques of cookingThe optimal technical condition in this cookingsystem is listed below: 11 % total alkali charge(Na2O), 0.6 alkali mass fraction, 160 °C maximumtemperature, 100 min the time at maximum tempera-ture, AQ charge 0.2 % and 1:7 mass ratio. The resultof cooking was achieved that pulp yield is 58.8 %and kappa number of pulp is 18.8. Variations of pulpyield and hardness depend mainly on the total alkalicharge and alkali mass fraction. TMAX and TC had asecondary effect on pulping process. AQ charge andmass ratio had only minor effect on the degree ofdelignification. Among these factors, KOH charge isthe most important factor which can reduce maxi-mum temperature and time at maximum temperaturewith increasing content of KOH.Comparison among Soda-AQ KOH-AQand KOH-K2SO3-AQ pulping of wheat strawFrom table 1 we can see, with the equal moleratio, pulp yield and hardness from KOH-AQ isbetter than that of soda-AQ. This shows that effi-ciency of delignification of K-base pulping ishigher than that of Na-base. Cooking system ofKOH-K2SO3-AQ can obtain satisfactory kappanumber and higher pulp yield. Burst index and tearindex in K-base pulping is better than that ofNa-base except higher pulp viscosity in K-basepulp. As a whole, properties of K-base pulp ex-celled or resembled to conventional Na-base pulp.Advantages of KOH-K2SO3-AQ system andutilization of K-rich black liquor in agriculture:AS-AQ (Alkaline sulfite-AQ) pulping has beenapplied widely in recent years and trends to substi-tute conventional Kraft pulping in straw-pulp in-dustry. Under comparable pulp intensity, higherpulp yield and lower kappa number are obtained.Pulp washing and bleashing is easier to in theAS-AQ pulping process than that of Kraft pulping.This is because the aldyde group of end of celluloseor hemicellulose is oxidated by sulfite which pre-vents peeling reactions during cooking and in-creases pulp yield.7Effluents of K-base cooking system are rich inpotassium and lignin which can be used organic andJ. QI-PEI et al., Cleaner Production of Wheat straw Pulp with Potash, Chem. Biochem. Eng. Q. 20 (1) 107–110 (2006) 109F i g . 3 – Effects of AQ and mass ratio on pulp yield andhardness during cooking (pulp yield: unscreened pulp yield;pulp hardness: expressed as kappa number)T a b l e 1 – Comparison between soda-AQ KOH-AQ andKOH-K2SO3-AQ pulping of wheat strawSoda-AQ KOH-AQ KOH-K2SO3-AQNaOH (w/%) 13 – –KOH (w/%) – 18 13K2SO3 (w/%) – – 10AQ (wAQ/%) 0.2 0.2 0.2Maximum temperature(TMAX/°C)155 155 160The time to maximumtemperature (t/min)60 60 60The time at maximumtemperature (t/min)60 60 100Mass ratio 7 7 7Unscreened pulp yield(yu/%)51.5 54.5 58.8Screened pulp yield(ys/%)39.2 42.7 45.5Kappa number 18.7 16.2 18.8Whiteness of rawpulp (w/%)51.7 54.5 47.8Pulp viscosity (mPa·s) 72.3 89.6 82.5Burst index (kPa·m2/g) 2.32 2.78 3.01Tear index (mN·m2/g) 5.05 5.25 5.33inorganic K-fertilizer. Conventional Kraft pulpingis little difference of solubility between Na2SO4 andNa2CO3(all about 45 g/100 g water at 100 °C) ingreen liquor. However, the solubility of K2CO3 andK2SO4 differs significantly (154 g/100 g water forK2CO3 and 20 g/100 g water for K2SO4 at 100 °Crespectively). It is easy to separate K2SO4 fromK2CO3 in green liquor according to difference ofsolubility. Not only can it form K-fertilizer(K2SO4), but also K2CO3 can be converted intoK2SO3 by reacting with SO2 or KOH with the pro-cess of causticization, which can avoid secondarypollution of white mud from sulfate.Lignin is the second natural reuseable organiccompounds, just behind cellulose. It is hard to bedecomposed by microorganism owing to its com-plex tredimension network structure and its highC/N ratio(C/N250).8 Lignin is also excellent melio-rating reagent of soil due to its in the formationhumic substances in soils.8 So lignin can act as inertcarrier for slow-released fertilizer because it can bedegraded by microorganism slowly to release nutri-ents. Lignin is the main component in black liquorthrough delignification from wheat straw. At pres-ent how to convert black liquor into organic fertil-izer has been becoming studying hotspot.In the course of study of converting black li-quor into organic slow-release fertilizer, (NH4)2SO3and NH3 pulping were applied.9 But these methodshas vital defect that cooking pulp was hard to bewashed and bleached. Properties of K-base pulp areexcelled or similar to traditional Na-base pulp.Study on K-base has risen up in recent years due toits preponderant recovery system than Na-basepulping system.1~4 From Table 2 we can see therewere rich potassium and lignin in K-base black li-quor, gentle pH in effluents of KOH-K2SO3-AQpulping system can use it irrigate directly. Effluentsfrom K-base cooking also can be evaporated andconcentrated to 40 % solid content by multi-effectevaporator. After ammoxidation and granulation, itcan form high-value organic compound fertilizerwhich contain 20 % of potassium(K2O), 25 % of or-ganic nitrogen and 45 % of organic matters. Thiskind of fertilizer can be senior commodity due to itsvirtue of slow-release effect. Those K-fertilizer (or-ganic fertilizer and K2SO4) can preferentially be ap-plied to Cl-fear crop such as tobacco and fruit. Itcan eliminate pollution from black liquor, at thesame time K-fertilizer can relieve high-cost fromexpensive K-base cooking reagent. The environ-mental pollution from black liquor could be elimi-nated after serious further development work. Thispulping system just is suitable to small-and me-dium-size paper mills which vastly dispersed inChinese country. An ecological cycling will beformed among crop, crop straw and paper mills.Conclusion(1) The optimal pulping conditions are as fol-lows: total alkali charge 11 % (Na2O), alkali ratio0.6, the maximum temperature 160 °C, cookingtime at the maximum time 100 min (mass ratio 1 : 7)and AQ charge 0.2 %. Pulp yield of this pulpingsystem is 58.8 %; kappa number is 18.8.(2) An important advantage of this pulping sys-tem is that the black liquor contains rich nutritionsuch as potassium and lignin that could be used asfertilizer resources for agricultural production.R e f e r e n c e1. Wong. A., Ng. D, Hull. J, Frederick, W. J., Tappi Proceed-ings 1989, 477.2. Wong, A., Derdall G., A novel sulphite pulping and chemi-cal recovery system for small-and medium-sacale pulpmills, Pulp Ppaper Canada, 92 (1991) 36.3. Tichy T., Wong A., Sulphite pulping of aloizia falcate fromEast Kalimantan, vol 2, Tropical Pulp conferences,Jakarts, 1991, p. 328~348.4. H’ng, 4th National Seminar on Utilisation of Oil PalmTree-Oil Palm Residues-progress towards Commericiali-sation, Malaysia,1997, 34.5. TAPPI Standard Fall, 1983.6. Tomkinson, J., Separation Purification Technology 24(2001) 529.7. TAPPI Pulp and Paper Science and Technology Volume IC. Earl Libby, Pulp p160~240.8. Meier, D., Schiene, R., Bioresource Technology, 49 (1994)121.9. Shen, D., Wu, J., Ecological Engineering 11 (1998) 121.10. Ren, X., Journal of Cleaner Production. 6 (1998) 349.110 J. QI-PEI et al., Cleaner Production of Wheat straw Pulp with Potash, Chem. Biochem. Eng. Q. 20 (1) 107–110 (2006)T a b l e 2 – Comparison of black liquor between KOH-AQand KOH-K2SO3-AQ pulping of wheat strawKOH-AQ KOH-K2SO3-AQpH 11.2 9.5Solid matters (/g l–1) 60.12 58.35Baume degree 6.1 5.3CODcr (/mg l–1) 62741 55019BOD5 (/mg l–1) 9113 7640Solid suspend (/mg l–1) 824 798Organic matters (/g l–1) 41.5 40.1Effective alkali (/g l–1) 2.24 0.75K2SO3 (/g l–1) – 1.54Lignin (/g l–1) 29.5 27.3Total potassium (K2O (/g l–1)) 16.16 16.88

Cleaner production of wheat straw pulp with potash

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