Etrinjit ile sülfat çöktürmesinin incelenmesi

Abstract

Bu çalışmada, atıksularda kimyasal çöktürme ile sülfat giderimi için alternatif bir az çözünen tuz olabileceği düşünülen etrinjitin sulu çözeltilerdeki davranışı incelenmiştir. Birinci bölümde yapılan çalışmanın önemi vurgulanarak, amaç ve kapsamı açıklanmıştır. ikinci bölümde, sülfat parametresinin önemi belirtilmiş, kaynakları belirlenmiş ve mevcut standartlar verilerek arıtma ihtiyacı ve yöntemlerinden bahsedilmiştir. Üçüncü bölümde, etrinjit ile ilgili literatür incelemesi yapılarak, etrinijt oluşumu, çözünürlüğü ve çözünürlüğe girişim yapabilecek maddeler belirlenmiştir. Dördüncü bölümde, etrinjitin sulu çözeltilerdeki çözünürlüğünü inceleyebilmek amacıyla sistem tanımlan yapılmıştır. Tanımlan yapılan sistemlerin, literatür araştırmalarının ışığında, su kimyası kavramları ile teorik çözümü yapılmıştır. Beşinci bölümde, yapılan teorik hesapların güvenilirliğinin kontrolü amacıyla gerçekleştirilen deneysel çalışmaların sonuçlan verilmiş ve değerlendirilmesi yapılmıştır. Son bölümde, hem teorik çözümler hem de deneysel sonuçlar ele alınarak, bunların değerlendirilmesi yapılmıştır. Teorik çözüm ile, stokiyometrik oranlarda bile yüksek sülfat giderme verimi elde edildiğinden, etrinjitin sülfat çöktürmede etkili olarak kullanılabileceği sonucuna varılmış olup, yürütülen deneylerdeki esaslar çerçevesinde bu sonuç deneysel olarak da gerçeklenmiştir.

Sulfate is an important parameter in wastewaters in terms of its impact on both sewer systems, treatment applications and receiving media. Therefore it is included constantly in the pretreatment regulations and in some of the 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. Many of the ago-industries, such as textile and fermentation as well as other industry categories discharge considerable amounts of sulfate into the environment. The treatment of sulfate can either be carried out by membran processes or by chemical precipitation. Membrane processes are generally difficult to apply in industrial wastewater 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. Chemical precipitation is based on formation of sulfate salts of low solubility. Calcium and barium are the two most common agents employed for precipitation. Sulfate precipitation as calcium sulfate is advantageous in many aspects such as ease of application and sludge removal. The main drawback of the process is the high solubility of calcium sulfate and existence of interfering ions in wastewaters. The standard for pretreatment of sulfate is mostly determined to be less than 1000 mg/1 which in most cases is exceeded by calcium sulfate solubility. Therefore this process can only be applied for source-based treatment of sulfate. Barium sulfate precipitation is very efficient, however, is applied rarely due to its limitations such as cost and toxicity. Other sparingly soluble salts of sulfate such as lead, are used only in special cases. Ettringite is a double salt which consists of calcium, sulfate and alurninium with a ratio of 6:3:2. Its formula can be given as CaeAkOoJCaSOi^KfcO. Ettringite phase is a regular constituent of hydrated cement pastes. It is formed from the tricalchim aluminate phases of the clinker and calcium sulfate at the beginning of the hydration process. The formation of etringite is believed to be responsible for expansion phenomena in most expansive cements with excessive SO4 content or in those being in contact with groundwaters containing large amounts of sulfates. Since ettringite crystallized in situ has a larger volume than the parent phases, the generated stress may result in expansion, cracking or, ultimately disintegration. Recent application of cement include the immobilization of radioactive and toxic wastes. It is important to study the mechanisms for sulfate swelling in concretes used as barrier material and waste conditioning material in repositories for Low- and Intermediate Level radioactive waste. Therefore,ettringite is a well-known compound for cement and concrete researchers. However, as they are interested in sulfate in pore solutions, where the solid/water ratio is much higher compared with aqueous solutions, the aquatic chemistry of ettringite is not adequately defined. To study the principles of ettingite precipitation for sulfate ions from wastewaters, it is important to review the aquatic chemistry of ettringite formation. In aqueous solutions, solid/water ratio is much lower, and it is possible, that the reactions, taking place in pore solutions of concrete, do not occur in aqueous solutions. As sulfate removal by ettringite precipitation is based on formation of salts of low solubilitiy, the most important thing for studying the ettringite precipitation, is to have the valid solubility product of ettringite in aqueous solutions. Through a wide literature survey, the solubility product of ettringite has been chosen to be 5.01 1E-46. The low solubilities due to the small solubility product of ettringite, make the system relatively independent from the ionic strength, compared with systems where other salts of low solubilities are employed. There were three systems chosen to study the equilibrium equations as well as interfering substances for ettringite precipitation. These systems were: 1. Al2(S04)3-Ca(OH)2-H20 2. A1C13-H2S04- Ca(OH)2-H20 3. Na2S04-AlCl3- Ca(OH)2-H20 The Aİ2(S04)3-Ca(OH)2-H20 system allowed us to check the the solubility product where no interfering ions were present. For theoretical solution of the system, the unknown species were determined and for 8 unknown species, 8 equations were written. Po is the amount of ettringite precipitated. 1. Unknown species: Ca2+, Al3+, S042', OH", CaOH*, CaSO40, Al(OH)4", P0 ; 8 species 2. Equilibrium equations:. Solubility product of ettringite Ksp: [Ca2+]6 [S042"]3 [OH"]4 [Al(OH)4"]2 XI . Complex formation reaction of CaOH* Ca2+ +OHT -> CaOH+ ;Ki. Complex formation reaction of CaS04° Ca2+ + S04"2 -> CaS04° ;K2. Complex formation reaction of Al(OH)4" Al3+ + 4 OH" -> Al(OH)4- ^3 3. Mass balances. Sulfate containing species : C,t.so4 = [SO42]+[CaSO40]+ 3 P0. Aluminium containing species : C, t,ai = [Al3+] + [Al(OH)4-] + 2 Po. Calcium containing species : CT,ca = [Ca2+] + [CaOH4] + [CaS04°] + 6 P0 4. Charge balance 2 [Ca2+] + [CaOH*] + 3[A13+] = 2[S042"] + [Al(OH)4 ] + [Off] The mass balance equations were written in terms of species in solution and a precipitate " Po". Theoretical solutions were based on the mass balances in that every solid phase to be precipitated was denoted as P. The solid phases assumed not to have precipitated were checked using solubility products. When Ca:S04:Al ratio was stoikiometric to ettringite, a pH of 11.01 was calculated from the theoretical solution of the system. The residual sulfate concentration was approximately 76 mg S0427L Five sub-systems were choosen to study the effects of pH and common ion. To raise pH above 1 1.01, excess Ca(OH)2 or NaOH was used. To study in low pITs, HC1 is added to the system. The same systematic was used in all system solutions and the critical points at the phase transformations are determined. Xll For the Al2(S04)3-Ca(OH)2-H20 system, which is defined to study of ettringite precipitation in aquaeous solutions, the theoretical results are given below: 1. There is apH range were ettringite precipitates and this pH range is determined to be 9.7-12.6. 2. Above pH : 12.6, monosulfete precipitation also occurs. 3. Below the pH, determined to be the lower limit for ettringite precipitation, ettringite is transformed to Al(OH)3 and sulfate removal efficiency decreases. 4. From the pH:9.3 downwards, the drop in pH results in precipitation of CaS04 and Al(OH)3. AlCİ3-H2S04-Ca(OH)2-H20 and Na2S04-AlCİ3-Ca(OH)2-H20 systems are defined to study the precipitation of sulfate from wastewaters. As the sources of sulfate and aluminium are different, these two systems enable us to determine which aluminium salt to use for ettringite precipitation. H2SO4 in the A1C13-H2S04- Ca(OH)2-H20 system and Na2S04 in the Na2S04-AlCl3- Ca(OH)2-H20 system are the two common forms of sulfate found in industrial wastewaters. Therefore, these two systems were representative in terms of sulfate source and coexisting substances. Theoretical solutions of these two systems showed the same trend of phase transformations as in the Al2(S04)3-Ca(OH)2-H20 system. A pH range of 9.5 -12.7 was determined to have only ettringite as solid phase and the theoretical residual sulfate concentration were below 70 mg/1 for pH values above 1 1. In AlCl3-H2S04-Ca(OH)2-H20 and Na2S04-AlCl3-Ca(OH)2-H20 systems, expecially when dealing with extremely high or low pH"s which correspond to addition of a large amount of either a base or an acid and where ettringite is not precipitated or is not the only solid phase, ionic strength is to high to use the theoretical resoults without a ionic strength correction. Experiments were planned to verify the theoretical results. The critical points where the phase transformations occur have been selected as key points for making up the program. The systemsimulated closed conditions to avoid interferences through the atmosphere. The solubilities corresponding to phase transformations could be reflected by the experimental results except for the aluminium hydroxid-ettringite precipitaiton which convert to ettringite within the interval of analitical sensitivity. xm The experimental results were approximately 20-30 mg/1 below the calculated values. That can be explained as the effect of adsorption of sulfates on solid phases existing in solution. The results of the study indicated that ettringite precipitation provides an alternative to calcium sulfate precipitation and satisfies the needs of pretreatment regulations if the precipitation conditions are well planned. The main advantages of this process are its higher efficiency and its relative independence from the ionic strength. However, the kinetics of ettringite formation was not studied and delayed crystalline formation can be a disadvantage of this process.