Emission of dioxines due to dressing facilities in the precious metal industry. Causes and possibilities for emission reduction

A great variety of waste-products and residuals of the precious metal industry are burnt in dressing facilities. The aim of the burning process is to recover the remaining precious metals. In some plants samples of filter dust were collected which contained very high concentrations of polychlorinated dioxins and furanes (PCDD/PCDF). To find explanations for this phenomenon and simultaneously to show possible countermeasures to reduce the PCDD/PCDF emissions, one of these dressing processors was investigated more intensively. This paper will show that the PCDD/PCDF-formation depends on the content of chlorine and sulphur in the primary materials and to an even larger extent on the content of combustible compounds. Very high levels of unburnt hydrocarbons are emitted when fuels with large calorific value (e.g. plastics) are burnt in the dressing oven. This is due to the uneffective control of the gas-fired burners and do the fact that secondary air injection is technically not possible. The extent of the PCDD/PCDF formation depends on the S/Cl-ratio in the cooling area. In order to improve the burnout of the flue gas several primary measures have been realised which has brought about a considerable reduction of PCDD/PCDF emissions.