Kimyasal çöktürme ile sülfat giderilmesinde yaklaşımlar ve uygulama esasları

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Tarih
1995
Yazarlar
Kabdaşlı, Işık
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
Bu çalışmada, endüstriyel atıksulardaki sülfatın kimyasal çöktürme yöntemi ile arıtılması konusu incelenmiştir. Birinci bölümde, yapılan çalışmanın önemi vurgulanarak, amaç ve kapsamı açıklanmıştır. İkinci bölümde, sülfat içeren atıksuların özellikleri ve kaynakları incelenerek, deşarj standartları verilmiş ve arıtma ihtiyacı saptanmıştır. Kimyasal çöktürme ile sülfat arıtımına etkili faktörler belirlenerek bunlar ile ilgili ayrıntılı bir literatür incelemesi yapılmıştır. Üçüncü bölümde kalsiyumla sülfat çöktürmesine baz teşkil edecek ve iyonik güç faktörünü dikkate alan teorik bir model kurulmuş; BaCOg ve Pb(NOg)2 ile sülfat çöktürmesi için de teorik hesap yaklaşımları verilmiş; baryum, kurşun ve stronsiyum tuzlarının kullanıldığı kaynak bazında kimyasal çöktürme sonrası geri kazanıma yönelik sistem tanımlan yapılmıştır. Dördüncü bölümde, kurulan teorik model ve hesap yaklaşımlarının güvenirliğinin araştırılması ve kimyasal çöktürme uygulama esaslarının belirlenmesi amacıyla gerçekleştirilen deney sonuçları verilmiş ve bunların değerlendirmesi yapılmıştır. Beşinci bölümde, teorik ve deneysel çalışma sonuçlarının değerlendirmesi yapılmıştır. Kalsiyumla kaynak bazında kimyasal çöktürmeye etkili faktörler iyonik güç, NaCl konsantrasyonu ve yabancı iyonların özellikle alkalinitenin mevcudiyeti olarak belirlenmiş; bu faktörlerden iyonik güç ve NaCl konsantrasyonu dikkate alındığında, geliştirilen teorik modelin kullanımı ile oldukça duyarlı sonuçların elde edilebileceği görülmüştür. Sonuç olarak sülfatın, kaynak bazında arıtımında makul aşırı kalsiyum dozajları kullanılarak 1000 mg/1 1er seviyesine indirilebileceği gösterilmiştir. Baryum tuzlarının kullanımı ile etkin bir sülfat arıtımının ve çöktürme sonrası sülfatın geri kazanımının mümkün olduğu gösterilmiş; baryumun geri kazanımı için de bir yaklaşım önerilmiştir. Kaynak bazında sülfat çöktürmesinde kurşun ve stronsiyum tuzlarının kullanılması durumunda gerek sülfatın gerekse çöktürme vasıtasının pratik olarak %100 ünün geri kazanılabileceği gösterilmiştir.
Sulfate is an important parameter in the wastewaters in term of its impact on both sewer systems, treatment applications and receiving media. Therefore it is included constantly in the pretreatment regulations and in the some of direct discharge standards. Sulfate initiated corrosion in concrete sewer system is a major problem caused by sulfate containing wastewaters. On the other hand, increased sulfate concentration in the discharges is often associated with high salinity which adversely affects many beneficial uses especially irrigation and drinking. The treatment of sulfate before irrigation and drinking purposes does not seem to be feasible. Many of the agro-industries, such as textile and fermentation as well as other industry categories, discharge considerable amount of sulfate into the environment. Although sulfate levels have not been frequently limited in direct discharge regulations, sulfate is a standard parameter of pretreatment regulations. Sulfide corrosion due to sulfate has been tried to be alleviated providing dilution where applicable. The treatment of sulfate can either be carried out by membrane processes or by chemical precipitation. Membrane processes are generally difficult to apply in industrial waste treatment. These processes are also costly if they do not serve materials or water reclamation. On the other hand sulfate removal by chemical precipitation is not fully evaluated for its potential application for sulfate containing industrial wastewaters. This study aims to investigate the theoretical bases of sulfate removal by chemical precipitation using calcium, barium, lead and strontium. The recovery of sulfate and regeneration of the precipitation agent is also studied. Chemical precipitation is based on the formation of sulfate salts of limited solubility. CaS04 has a relatively high solubility and may not be appropriate for the precipitation of wastewaters containing low level of sulfate. Sulfate concentration in the wastewaters of above mentioned industries are low or moderate. However, the source of sulfate in many of these industrial processes are highly concentrated batches or flows in terms of sulfate. Therefore segregation of these flows and a source-based sulfate treatment using calcium can be an effective solution. On the other hand application of most effective precipitation on these sources using barium, lead and strontium can be applicable if sulfate recovery as well as regeneration of these agents can be achieved. These salts are also effective for the treatment of wastewater containing low level sulfate with or without recovery. Therefore in this study calcium precipitation is considered for high sulfate precipitation. Barium, strontium and lead XIV are evaluated as far as their effectiveness and recovery of sulfate and/or initial settling agents are concerned. A literature survey was made to evaluate the factors such as;. ionic strength,. NaCl concentration,. interfering substances,. temperature,. solid phase,. oversaturation on the precipitation with calcium, strontium, barium and lead. Ionic strength and NaCl concentration were determined as the main factors affecting CaSCs solubility. The solid phase at the conditions of this study was determined as gypsum. A theoretical approach using the proposed models and experimental data reported in the literature was developed. The model can be explained as follows.. -A I z^z2 I JÎ (0.06 + 0.6BB) I zxz2 I J. logy* - - - - + - +B"I-Eln{±+a2I) WT (i+T^): zxz2~\ 2 logy0 - -0.17 yT Gypsum solubility has been observed to reach the maximum at 2.5 m NaCl. Only magnesium and some organic ligands have been determined to affect on the calcium sulfate precipitation. The effect of temperature on the gypsum solubility was practically negligible. A theoretical evaluation considering the literature data was made. A system of sulfate recovery from barium sulfate solution which is based on the formation of Ba^PO^ was developed as follows. 3BaS04 + 2Na3P04 -> Ba3(P04)2 + SNa^C^ for pH > 13 Recovery of barium from Ba^PO^ by Ca5(P04)3OH precipitation was proposed. For the applications of lead and strontium precipitation, theoretical evaluation was made and precipitated sulfate recovery and lead or strontium regeneration processes were delineated as represented by lead as follows.. Sulfate precipitation; PbC^ or Pb(N03)2 + NajSC^ - PbS04 + 2NaNOs. Sulfate recovery; xv PbS04 + NajCC^ -+ Na2S04 + PbC03. Lead regeneration; PbC03 + HC1 or HN03 -* PbC^ or Pb(N03)2 + C02 (g) The laboratory experiments were carried out for the verification and demonstration of developed theoretical approaches and recovery and regeneration processes. Results were compared with literature data. The results of the study with the main conclusions drawn can be summarized in the following scheme. The main results of calcium precipitation can be given as follows.. In general, the modelling approach depicted in Section 3.2 is determined to be a relatively easy to use and accurate enough for the prediction of calcium sulfate precipitation processes. The experiments carried out in order to check the reliability and accuracy of the model yielded following results.. 60000 mg/1 sulfate containing textile dyeing bath which represents the highest ionic strength encountered in the wastewaters (I = 1.75-1.88) together with synthetic samples equivalent to the baths composition except for the dyes and organic components is precipitated using CaCL and results indicated an excellent fit to the model as well as the absence of significant inhibition of precipitation due to impurities existing in the bath. In order to see the effect of NaCl concentration at 2.5 m level the same bath composition together with synthetic sample are added NaCl and precipitation experiments are conducted using CaCL. Results also showed a close fit to the theoretical approach.. Textile dyeing baths and parallel synthetic samples containing 20000 - 30000 mg/1 sulfate are precipitated with CaCl^ Results are 650 - 850 mg/1 higher than those of predicted by the model which can be explained by CaCOj precipitation with the alkalinity content of water which is due to longer equilibrium durations required at these concentration.. The experiments carried out at 10000 mg/1 sulfate level using textile dyeing bath and synthetic samples indicated that synthetic sample response is in accordance with the model. However, the real sample yielded higher sulfate concentration which is explained by kinetic factors.. Kinetic limitations are attempted to be evaluated by seeding and conducting precipitation experiments using excess calcium both fed to the precipitation solutions which achieved equilibrium. Experiments indicated no or limited benefit of seeding to shorten the equilibrium duration not affected excess dosages significantly precipitation rate. xvi . Precipitation experiments using reasonable excess calcium indicated effective results by which 60000 mg/1 sulfate could be precipitated down to 1500 mg/1 for real and synthetic samples.. Increased efficiency of precipitation as compared to the data reported in the literature with increasing temperature was observed with the experiments at 30 - 45 °C using 60000 mg/1 sulfate dyeing bath which is evaluated as an advantageous point for the precipitation of hot dyeing baths.. The experiments conducted with copper plating baths containing 13000 mg/1 sulfate yielded 1700 mg/1 sulfate with Ca(OH)2 precipitation which is in accordance with theoretical calculation. Results and evaluation of experiments using barium, lead and strontium for precipitation and recovery of sulfate as well as reuse of precipitation agents are summarized as follows.. High efficiency of BaCL was tested experimentally using metal plating wastewater containing 3000 mg/1 sulfate. On the other hand problems associated with the use of BaCO- were theoretically determined and experimentally demonstrated using textile wastewater with 2000 mg/1 sulfate and parallel synthetic samples. Sulfate concentration as high as 900 mg/1 were obtained which is explained by the dissolution limitation of barium carbonate.. Precipitation and recovery applications using BaCU were conducted with sulfate dyeing bath containing 27000 mg/1 sulfate. Precipitation with stoichiometric barium removed the sulfate practically completely from the wastewater. At the second stage precipitated BaSO^ was treated with Na^PO, at high pH to convert the barium sulfate to Ba^POA» complete conversion required excess phosphate dosage. Kinetic evaluations of this conversion were also made. Following the separation of solid Ba^PO*)^ phase, the solution was treated with lime to remove the excess phosphate as Ca^PO,^ to obtain the practically pure solution of Na^SO-. At the third stage a procedure was defined for recovery of the barium from which foresees the binding of phosphate into Ca5(P04)3OH using Ca(OH)2-. Sulfate precipitation using PtyNO.,^ was demonstrated by the experiments on metal plating wastewater containing 3300 mg/1 sulfate and practically all sulfate was removed.. Treatment of concentrated sulfate solutions especially when solution contains alkalinity poses problems due to complexation of lead-nitrate and lead carbonate precipitation limitations. A theoretical modelling involving the system components was developed and experimentally checked. The experiment using 27000 mg/1 sulfate containing textile dyeing bath resulted in complete removel as predicted by the model. The application of PbC^ instead of Pb(NCX,)2 in the same experiment removed practically all the sulfate, however, kinetic limitation in terms of precipitation duration was observed. xvu . Recovery of sulfate from PbSO, solid was experimentally achieved with quantitavely using Na2COj.. At the third stage lead was recovered by acidifying PbCO., solid with HC1.. A similar system and procedure as with lead was defined for strontium and theoretical and experimental verifications were made. The results indicated very high efficiencies of both sulfate removal and recovery as well as regeneration of strontium. An additional benefit of this system is the high solubility of SrCL which facilitates initial precipitation.
Açıklama
Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1995
Thesis (Ph.D.) -- İstanbul Technical University, Institute of Science and Technology, 1995
Anahtar kelimeler
Sülfat, Çöktürme, Sulfate, Percipitation
Alıntı