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Chelating Agents – Complexing Agents [3–5]
The presence of heavy metals negatively influences many production processes and paper properties. In particular, they interfere with the bleaching agents in chemical and mechanical pulp production, reducing their effect, so the task of chelating agents is to counteract this detrimental impact. Chelating agents can also reduce or stop uncontrolled decomposition of a hydrogen peroxide solution, which would result in loss of bleaching effectiveness.
Chelating agents that contain amino and carboxyl groups mask metal ions effec¬tively. Chemical compounds of this type are nitrilotriacetic acid (NTA), ethylene¬diaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA). Other complexing agents include the soluble salts of oxalic acid, citric acid, tartaric acid, gluconic acid, amines, and ammonia.
Both the demand for increased brightness and the technology trend to elemental chlorine-free and total chlorine-free bleached pulps has led to an increased use of chelating agents today, dominated by DTPA and EDTA.
In view of environmental protection, there exists a controversial discussion about the biodegradability of chelating agents. The results of newer investigations show that, under certain conditions, EDTA and, with some restrictions, DTPA, are fairly biodegradable. Their degree of biodegradability is still under discussion. It is strongly influenced by pH (optimum at pH 8.5–9), by ultraviolet light and by the effectiveness of adapted microorganisms. Up to 90 % biodegradability of EDTA is reached in some pulp and paper mills.
Of less importance are chelating agents based on polyphosphates, phospho-nates, and hydroxycarboxylates as well as nitrogene free products like citrate, ta¬trate and gluconate.
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