Razvoj metoda za analizu halogenih elemenata u čvrstim uzorcima u atmosferi kiseonika

Abstract

Analiza kvaliteta ĉvrstih materijala, u zavisnosti od namene, obuhvata: tehniĉku analizu, elementarnu analizu, analizu makrokomponenata i mikrokomponenata. Poseban analitiĉki zadatak predstavlja analiza anjona u sloţenoj matrici ĉvrstog uzorka. U literaturi i analitiĉkoj praksi postoji veliki broj metoda za odreĊivanje halogenih elemenata, od klasiĉnih do savremenih (instrumentalnih), koje se razlikuju kako po principima rada, tako i po kriterijumima koji karakterišu analitiĉke tehnike, od kojih treba izdvojiti taĉnost i reproduktivnost. Predmet rada ove doktorske disertacije su halogeni elementi u uglju. Halogeni elementi u uglju su prisutni u niskim koncentracijama, ali je uticaj ovih elemenata na ţivotnu sredinu i zdravlje ljudi od posebnog znaĉaja. Istraţivanja u okviru ove disertacije obuhvataju analizu halogenih elemenata u uglju, rezultate istraţivanja pirohidrolitiĉke metode za odreĊivanje halogenih elemenata, kao i primenu eksperimentalnog dizajna za razvoj analitiĉkih metodologija. Cilj ove doktorske disertacije bio je da se utvrdi metodologija i odrede uslovi za istovremeno odreĊivanje fluora, hlora i broma u uglju. U okviru ove disertacije primenjene su statistiĉke metode za organizaciju i izvoĊenje eksperimenata, i tehnike za obradu i analizu dobijenih rezultata. Primenom frakcionisanog faktorijskog dizajna ispitan je uticaj šest promenljivih na sagorevanje uglja u kiseoniĉnoj bombi i apsorpciju gasovitih jedinjenja koja se tokom sagorevanja izdvajaju iz uglja. Ispitan je uticaj poĉetnog pritiska kiseonika, dodatak katalizatora i pomoćnog goriva na sagorevanje uglja, kao i tip apsorpcionog rastvora, dodatak redukcionog agensa i vreme hlaĊenja bombe nakon sagorevanja na apsorpciju gasovitih jedinjenja. Koncentracija jona u apsorpcionom rastvoru merena je potenciometrijski jon-selektivnom elektrodom (za fluorid-jon) i jonskom hromatografijom. Rezultati frakcionisanog faktorijskog dizajna su analizirani pomoću Pareto-grafika, grafika glavnih efekata i dijagrama interakcije parametara. UtvrĊeno je da se dodatkom katalizatora i povećanjem pH vrednosti apsorpcionog rastvora smanjuje koncentracija halogenid-jona u apsorpcionom rastvoru. S druge strane, povećanjem poĉetnog pritiska kiseonika i dodatkom pomoćnog goriva, povećava se izdvajanje halogenih elemenata iz uglja tokom sagorevanja u atmosferi kiseonika. Na osnovu rezultata preliminarnih istraţivanja, pritisak kiseonika i zapremina pomoćnog goriva su detaljnije ispitani primenom metodologije odgovora površine. Rezultati optimizacionih eksperimenata razmatrani su analizom grafika odgovora površine, procene regresionih koeficijenata i analize varijansi. Pošto su se optimalni uslovi za odreĊivanje fluora i hlora razlikovali, definisan je grupni parametar - ukupni halogeni u uglju (engl.- total halogen in coal, TCX). Ukupni halogeni u uglju su izraţeni preko hlora, a hlor je izabran iz razloga što predstavlja dominantan halogeni element u uglju. Optimalni uslovi metode odreĊivanja ukupnih halogena u uglju sagorevanjem u kiseoniĉnoj bombi su: pritisak kiseonika od 2,5 MPa, zapremina pomoćnog goriva od 150 µL, apsorpcioni rastvor je voda uz dodatak 0,5 % rastvora vodonik-peroksida i hlaĊenje kiseoniĉne bombe u ledenom kupatilu 15 minuta nakon sagorevanja uglja. Masa uzorka je bila 0,5 g. Validacija predloţene metode za odreĊivanje fluora i hlora sagorevanjem u kiseoniĉnoj bombi uraĊena je analizom internih referentnih uzoraka uglja i sertifikovanih referentnih ugljeva (BCR 460 i NIST 1632c). Relativna standardna devijacija odreĊivanja fluora bila je manja od 5,0 %, a procenat izdvojenog fluora u odnosu na referentnu vrednost veći od 95 %. Relativna standardna devijacija odreĊivanja hlora bila je manja od 6,0 %, a procenat izdvojenog hlora veći od 85 %, što znaĉi da je odreĊivanje fluora i hlora predloţenom metodom precizno i reproduktivno. Predloţenom metodom je odreĊen i sadrţaj sumpora u uglju. Validacija metode za odreĊivanje sumpora je pokazala da je standardna devijacija merenja manja od 6 %, uz dobru ponovljivost.

Comprehensive analysis of the quality of solids, depending on the application, includes: technical analysis, elemental analysis, analysis of the macrocomponents and microcomponents. A special analytical task is the analysis of anions in a complex matrix of solid samples. In literature and in analytical practice there are a number of methods for the determination of halogens, from classic to contemporary (instrumental), which differ according to the principles of work, and according to criteria that characterize the analytical techniques of which accuracy and reproducibility should be allocated. The subject of the work of this doctoral thesis are halogen elements in coal. Halogen elements in coal are present in low concentrations, but the impact of these elements on the environment and human health is of particular importance. Research in this thesis are designed in the field of analysis of the halogen elements in coal, the results of pyrohydrolityc methods for the determination of halogens, as well as on the application of experimental design for the development of analytical methodologies. The aim of this doctoral thesis was to establish methodology and determine the conditions for simultaneous determination of fluorine, chlorine and bromine in coal. Within this dissertation statistical methods were applied for the plan and for conducting experiments, and techniques for processing and analysis of results. Applying fractional factorial design, the influence of six variables on coal combustion in oxygen bomb and absorption of gaseous compounds during combustion was investigated. The analyzed variables were oxygen pressure, catalysts, combustion aid, type of absorbent solution, reducing agent and bomb cooling time. The concentration of ions in the absorption solution was measured by ion-selective electrode (the fluoride-ion) and by ion chromatography. The results of fractional factorial design were analyzed using Pareto-graphics, graphics of main effects and interaction plots. It was found that with the addition of the catalyst and with the increase of pH value of the absorption solution the concentration of halide ions in the absorption solution decreases. On the other hand, increase of oxygen pressure and the addition of combustion aid, increases the separation of halogens from the coal during combustion in an oxygen atmosphere. On the basis of preliminary studies, oxygen pressure and combustion aid were examined in more detail by using response surface methodology. Results of the optimization experiments were discussed using response surface plots, estimateted regression coefficients and analysis of variance. Since the optimum conditions for the determination of fluorine and chlorine differed, the group parameter - total halogens in coal (TCX) was defined. Total halogen in coal are expressed in terms of chlorine and it was chosen because it is the dominant halogen element in coal. The optimal parameters for the determination of halogens in coal were 0.5 g of coal samples, oxygen pressure of 2.5 MPa, 5 cm3 of H2O as the absorption solution, 5 cm3 of H2O2 as reducing reagent, 150 µL of mineral oil as the combustion aid, and a cooling interval of 15 minutes after combustion. Validation of proposed method was performed by determination of fluorine and chlorine in in-house prepared reference coal samples and in certified reference materials (BCR 460 and NIST 1632c). The relative standard deviation of determination of fluorine was less than 5.0%, and recovery was greater than 95%. The relative standard deviation of determination of chlorine was less than 6.0%, and recovery greater than 85%.The method was sensitive, with good reproducibility and accuracy. The proposed method was applied for determination of sulfur in coal. Validation of a method for determining the sulfur content is shown that the standard deviation of measurements was less than 6%, with a good reproducibility. The limit of detection (LOD) and limit of quantification (LOQ) is estimated for ion chromatography measurements and standard deviation of determining ions in the mixed standard solution of the lowest concentration. It has been found that the LOD and LOQ for fluorine were 0.4 mg / kg and 1.26 mg / kg; chlorine 0.84 mg / kg and 2.79 mg / kg; sulfur 2.2·10-5 % and 7.2·10-5 %. The proposed method was applied for the determination of fluorine, chlorine and sulfur in Kolubara and Kostolac coal samples.