EMISSION OF DIOXINES DUE TO DRESSING FACI LITIES 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 proc ess is to recover the remaining precious metals. In some plants sample s of filter dust were collected which contained very high concentratio ns of polychlorinated dioxins and furanes (PCDD/PCDF). To find explana tions for this phenomenon and simultaneously to show possible counterm easures to reduce the PCDD/PCDF emissions, one of these dressing proce ssors 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 em itted when fuels with large calorific value (e.g. plastics) are burnt in the dressing oven. This is due to the uneffective control of the ga s-fired burners and do the fact that secondary air injection is techni cally not possible. The extent of the PCDD/PCDF formation depends on t he 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 bro ught about a considerable reduction of PCDD/PCDF emissions.