Exact knowledge on the behaviour of metal ions is needed to improve the efficiency and environmental aspects of modern pulp bleaching sequences, especially those including a hydrogen peroxide stage. The aim of this study was to extend this knowledge and it was based on practical phenomena and problems. First, the decomposition [I, II], thermodynamic chelating ability [I, II, III] and solubility [III] of chelating agents in simulated hydrogen peroxide conditions were investigated. Second, interactions between hydrogen peroxide and a typical construction material, titanium, were studied [IV, V] taking into account the influence of calcium and silicate inhibitors [V], as well as iron and manganese [IV, V]. The analysis of metals [VI-IX], pH [VIII], hydrogen peroxide and chelating agents ADA [X] and NTA [XI] formed the third part of the thesis.
Hydrogen peroxide anion was able to decompose ADA at a concentration level of 400 mg/l (60 ºC, pH 10-11, total hydrogen peroxide concentration 1000 mg/l). The average residual percentage was 71. In similar conditions 94% of DTPA remained. The residual percentage of hydrogen peroxide was 40 for both the chelating agents. As a conclusion, degradation of ADA may beneficially occur already in the industrial process before WWTP. Further, EDTA was proved to be more recalcitrant against chemical decomposition compared to ADA. The percentage of residual was 94 at a hydrogen peroxide anion level of 1200 mg/l (60 ºC, pH 10-11, total hydrogen peroxide concentration 5000 mg/l). Also decomposition of hydrogen peroxide was low, 74% remained when EDTA was the complexing agent. The reason for the observed higher decomposition of ADA compared to the other two chelating agents was probably the higher electron deficit in nitrogen or the existence of Mn(III) and Mn(IV) oxides. The observed high durability of EDTA supports aspirations for minimizing its utilization by using replacement ligands (ADA, S,S’-EDDS, hydroxycarboxylic acids) or by developing effective procedures for its removal.
The solubility of DTPA, EDTA and ADA in an alkaline hydrogen peroxide environment was limited but increased in the presence of transition metals, Fe and Mn. A possible explanation for the limited solubility is that the complexes, particularly the magnesium complex, are adsorbed on to magnesium hydroxide precipitate.
According to thermodynamic speciation in typical bleaching conditions, chelating agents exist mainly as calcium (70-80%), manganese (15-20%) and magnesium (0-5 %) complexes, which is also relevant in the experiments of this study. Iron is thermodynamically more difficult to chelate due to its strong self-hydrolysis. As a suggestion for further investigations, the correspondence with species distribution calculations could be investigated experimentally using LC, CE or polarographic methods.
The typical construction material of bleaching lines, unalloyed titanium Grade 2, corroded in alkaline magnesium containing hydrogen peroxide solutions when the HOO- anion level exceeded 200 mg/l. Vanadium and aluminum alloyed Grade 5 was inferior to Grade 2. The presence of calcium ions and silicate improved corrosion resistance by enhancing the critical anion levels. Proposed protection mechanisms for the observed inhibition were physical adsorption of the Ca2+ ions or Ca-silica compound as well as the formation of calcium peroxide near a metal oxide surface. The presence of iron and manganese further enhanced the critical levels; corrosion of Grade 2 was low, even at a hydrogen peroxide anion level of 800 mg/l. The corrosion potential of Grade 2 could be used as an indicator whether uniform corrosion of titanium took place. During rapid corrosion, potentials were under 200 mV (SHE), and clearly lower than the potentials of platinum.
A comparative study in pulp matrix between three different metal analytical methods, ICP-AES, XRF and ISE was performed. For the ICP-AES technique, investigations focused on the extraction of metals to aqueous solution by chelation or acid hydrolysis. Over 90% yields were obtained for Ba, Ca, Mg, Zn and, most importantly, Mn. Yields for iron and aluminum were matrix dependent, typically 40-60% and 15-40%, respectively. DTPMP appeared to be the best chelating agent, but overall the most effective extracting method was acid hydrolysis. A rapid X-ray fluorescence method with a quantification limit of 2-3 mg/kg was developed for Mn, Fe and Cu. As an undoubted benefit, direct analysis was successful. Dealing with ISE techniques, for Na+, K+, Ca2+ and Cl- quantification limits under 1 mg/kg were reached and some important sources of error were elucidated. pH measured in room temperature may be more than two units higher than the real values of the process. Atmospheric carbon dioxide dissolves in the measuring systems forming a carbonate buffer and decreasing the reversibility of the measurements. It also may precipitate some part of Ca2+ ions as carbonate. When an ion strength adjustor is not employed, variations in the total ionic strengths of samples must be taken into account. Where the adjustor is used, its adsorption on to fibres may lead to elevated high values.
Sensitive GC methods for ADA and NTA were developed. Detection limits in water samples were under 6 µg/l and reproducibilities were good, under 15% in distilled, lake and waste waters. However, the extraction step from sediments is to be improved. The procedure developed was suitable for simultaneous determination of DTPA, EDTA, ADA and NTA.