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Sülfat içeren atıksuların kalsiyun ile çöktürülmesine etki eden faktörlerin incelenmesi

Evaluation of factors affecting sulfate removal by calcium precipitation

  1. Tez No: 66852
  2. Yazar: DİLEK YILMAZ
  3. Danışmanlar: PROF. DR. OLCAY TÜNAY
  4. Tez Türü: Yüksek Lisans
  5. Konular: Çevre Mühendisliği, Environmental Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1997
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Çevre Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 97

Özet

ÖZET Bu çalışmada endüstriyel atıksulardaki sülfatın kalsiyum tuzlan kullanılarak kimyasal çöktürme yöntemi ile arıtılması konusu incelenmiştir. Birinci bölümde, yapılan çalışmanın önemi vurgulanarak, amaç ve kapsamı açıklanmıştır. İkinci bölümde, sülfat içeren atıksuların özellikleri ve kaynaklan incelenerek, deşarj standartları verilmiş ve arıtma ihtiyacı saptanmıştır. Üçüncü bölümde, sülfatın kalsiyum ile çöktürülmesinde çöktürmeye etki eden faktörler doğrultusunda kapsamlı bir literatür taraması yapılmıştır. Dördüncü bölümde, kimyasal çöktürmeye etki eden faktörlerden iyonik güç ve kompleks oluşumunun çöktürme mekanizması üzerine olan etkilerini göstermek amacıyla üç farklı sistem için teorik çalışmalar yürütülmüştür. İyonik güç düzeltmelerinde aktivite katsayısı hesabında çeşitli aktivite katsayısı bağıntılarının kullanılabilirliği araştırılmış ve elde edilen teorik çözümlerle karşılaştırılmalarına imkan sağlanmıştır. Sodyumun, sülfatla NaS04" kompleksi oluşturarak çözünürlükte meydana getireceği artış teorik olarak incelenmiş ve önce 0.4-0.45 M daha sonra 0.3-0.45 M iyonik güç aralıklarında bu komplekse bağlı olarak çözünürlüğü ifade eden teorik bir model geliştirilmiştir. Beşinci bölümde ise teorik çözüm sonuçlarının deneysel olarak elde edilebilirliğini görmek amacıyla sentetik ve gerçek numunelerle yürütülmüş olan deneysel çalışmalar yer almaktadır. Böylece stolriyometrik üstü kalsiyum ilavelerinde sülfatın 1000 mg/l'ler seviyesine düşürülebileceği gösterilmiştir. Altına bölümde bu çalışmayla varılan sonuçlar ve bunların değerlendirilmesi yer almaktadır. xı

Özet (Çeviri)

SUMMARY Within the rise in industrial consciousness of the society and the approach to the protection of the environment, the importance of industrial wastewater treatment has increased vastly. In various industry categories treatment of wastewater formed from the result of productivity, has been inadequate because of the insufficient capacity of technologies with the implementation of discharge standards. Therefore according to this development, new studies and progress should be considered for new treatment technologies. Treatment of wastewater containing sulfate can be given as an example for this situation. Especially, sulfate initiated corrosion in concrete sewer systems and therefore a pre- treatment of sulfate becomes inevitable within the pretreatment discharge standards. When a study for applicable technologies for sulfate removal is made, with advanced treatment, anaerobic biological treatment and chemical precipitation, satisfactory results can be obtained and among these treatment systems chemical precipitation becomes more important not only in application simplicity but also for economical cases. Chemical precipitation process is based on the formation of sparingly soluble salts of anions and cations. The important factors affecting this formation from the view point of thermodynamics and kinetics are as follows: * Ionic strength * Existance of interfering substances * Temperature * Type of precipitated solid phase * Factors related to precipitation process Within the above mentioned factors; especially the effect of ionic strength and sodium on solubility need to be investigated in the light of theoretical and experimental for CaS04 work. The main objectives of mis study in this content can be summarised as follows: First, it is tried to define which approaches will be used in sulfate precipitation for ionic strength correction for the critical moderate ionic strengths and what efficiency will be obtained in sulfate removal for moderate ionic strengths. XllSecondly, both for moderate and high ionic strengths, effect of observed but not defined NaS(V complex formation on solubility has been attempted to define within the range of 0.4-0.45 M (narrow) and 0.3-0.45 M (wide) ionic strengths. The nature of the calcium sulfate phase forming is a function of the solution temperature and pressure. CaS(>4 is also affected by the ionic strength of the aquatic medium and in the presence of foreign ions or compounds. In an ideal solution, where the components (e.g., the dissolved substances and the solvent) do not influence one another, the activities of the components are equal to their concentrations. In a real solution the activity of a component is not equal to its concentration due to interactions between the components. This makes it necessary to express the activity of the components as a product of the concentration and a term,called activity coefficient representing the deviation from the ideality at the given concentration. In the theoretical calculations in this study, mean activity coefficient of calcium sulfate is obtained with the following three approaches: Davies approach; AZ1Z9VI iogvT=- 1 + jî +0.1Z1Z2I Long -range electrostatic forces or ion atmosphere effects are descriibed for dilute solutions by the Debye-Hückel equation and Davies approach is a useful form of this equation. Pitzer specific ion interaction approach; r InTMx =kiZx|F + Tf-Em8 vM 2BMa+ZCMa+2~(l>Xa VX J v c V vx ?“) 2BcX+ZCcX+2^-«i,Mc +ZZmcmav 1(2vMzMCca +vMv|/Mca +vx\|/caX) +IImcnW-rVcffl+IIn».%-f-Vita'+:I«n.(vM^i +vx^nx) 2- o 2 1 + 1 r-I Bromley has independently developed a system of representation and estimation for activity coefficients of strong electrolytes. His system may be described as a simplification of the Pitzer model. The third virial coefficient is omitted, and avd the second virial coefficient is modified into a form with a single parameter B. Determination of the activity coefficients for K0.5, Davies and for IX). 5, Bromley approximation have been used and Pitzer's approximations has been taken into consideration only in one theoretical Na2SC«4 - CaCfe system calculation because of the lack of second and third virial coefficients. In order to determine and examine the change in the solubility of calcium sulfate, sulfate initial concentration of 10000, 8000 and 6000 mg/1 in the ionic strength range of 0.3-0.45 with NaS(V complex formation the following systems with or without Na, theoretical calculations have been made. From these, the first one is: 1) Na2S04-CaCİ2 system In this system in order to obtain the ionic strength of 0.3-0.5 range, for initial 10000, 8000 and 6000 mg/1 sulfate concentrations, theoretical studies have been carried on. The species in this system are: Ca2+, SO/, Na+, NaSCV, CaSO/, CI”and P. Pis the amount of CaS04 precipitated. Seven unknown species of the system have been solved by the help of equilibrium equations, mass balance and electroneutrality, ionic strength and mean activity coefficient expressions and y0 are as follows: xivEquilibrium equations: K8p=yJ[CB2+IsO|] = 10^ [NaSO;] [Na+]yT[sO^] K k u__in°-72 VofCaSoSl 28 CaSo:“ vltca^sor1 Mass balances: CT,Ca = [Ca2+] + [CaS02] + P Ct,so4 = [S04=] + [CaSOj] + [NaS04~] + P CT,Na=[Na+] + [NaS04-] cT,ci = [cr]. Electroneutrality: 2[ca2+] + [Na+] = 2[sO^~] + [NaSOi] + [cr Ionic Strength Expression: I = |(4[Ca2+] + 4[so2f ] + [Na+] + [d-] + [NaSC>4 ]) logyo = -0.17VF xvThe mean activity coefficient of the system is calculated based on Davies and Bromley approximations and as a result of theoretical considerations, by adding stoichiometric and excess Ca, a decrease in sulfate concetration has been observed from 10080 mg/1 to 1200 mg/1 after precipitation. 2) Na2S04-NaOH-CaCİ2-HN03 system This system examined the effect of increase in sodium concentration with NaS04”, by adding the excess sodium to Na2S04- CaCl2 system as NaOH. To fix the ionic strength, HNOs is added to the system. Ca2+, S04=, If, NaS04“, CaSO^Cl”, OIT, N03 and P are the species and similar equations as those of case have been employed. In order to observe the change in sulfate concentration by formation of NaS04“ with sodium, and to define that in the range of 0.4-0.45 M, two theoretical models are presented. The methods in these approaches are based on approximations for 0.4-0.45 M (narrow) ionic strength and for 0.3-0.45 M (wide) ionic strength. In both approaches: Ct,so4 = [S04=] + [CaSOj] + [NaS04_] In the above equation, for predefined ionic strengths average values for both S04~ and CaS04”have been calculated. For NaS04“ complex, however a mean value could not be determined. Therefore, theNaS04”complex is expressed as; CT,S04 =(aS04 +t>CaSO/) + f(NaS°4) which indicated a lineer function. I. approach was, Y = CT,S04 = (aS04 +bcaSO/) + f(CT,Na>NaS04J II. approach was, Y = CT,S04 = (aS04 +bCaSO;) + f(l,NaSOi) XVIIn the range of 0.4 so4 = 0.023602434 + 0.0187.1 3) H2S04-Ca(OH)2-HN03 system Theoretical solutions of the system H2SO4 - Ca(OH)2 - HNO3 which does not contain Na ions, was made for an ionic strength interval of 0.3-0.45 M. These solutions enabled us to evaluate the effect of NaSO_4 complex on sulfate solubility. The laboratory experiments were carried out on synthetic samples and textile dyeing bath wastewater for the verification and demonstration of the developed theoretical solutions and approximations. The systems simulated closed conditions to avoid interferences through the atmosphere. The results of the study with the main conclusions drawn can be summarised in the following scheme: XVI 1* For the synthetic samples containing 10080, 8000 and 6000 mg/1 sulfate in system Na2S04 - CaCl2 and H2S04-Ca(OH)2 -HNO3, the experimental results have shown an excellent fit to the theoretical calculations. * In H2SO4 - Ca(OH)2 - HNO3 system containing 10080 mg/1 initial sulfate concentration with the addition of excess Ca(OH>2, lower sulfate values than the Na2S04 - CaCl2 - H20 system are obtained. This situation is explained by the ionic strength and NaSCV complex formation effects. * According to the theoretical approaches, results have shown that at narrow and wide ionic strengths the NaSCV complex has been found to be the most important species which increasing the solubility of calcium sulfate. * For the 10042 mg/1 sulfate containing textile dyeing bath, using %50 excess CaCl2 and at pH 6- 6.5, sulfate removals obtained as %70 to %79 in the experiments. Results are 270 and 600 mg/1 higher than those predicted by the theoretical calculations respectively which are attributed to longer duration required for equilibrium at these concentrations. * 8000 mg/1 sulfate containing textile dyeing bath has been precipitated using CaCl2 and results indicated an excellent fit to the theoretical calculations as well as no significant inhibition of precipitation due to impurities in the bath has been observed. * The experiments carried out at 6000 mg/1 sulfate level using textile dyeing bath yielded higher sulfate concentration than those of theoretical calculations which are explained by kinetic factors. Using %50 excess CaCl2 dosage sulfate could be precipitated down to 1504 mg/1 which fitted the theoretical calculations. XVI 11

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