The Determination of Mercury Source Emissions in The Presence of High Levels of SO₂

Abstract

Existing reference methods for the determination of mercury emissions from stationary sources include collection using acidic iodine monochloride 1 and acidic permanganate. 2 In both cases collection is preceded by appropriate removal from the stack and is followed by analysis for mercury using flameless atomic absorption.3 Collection of mercury vapor by the absorbing solution is dependent both on the acid content and the oxidizing power of the solution. 4 If there are components in the gas stream that significantly alter the nature of the absorbing solution the collection efficiency may be seriously degraded. Thus, both acid-base and redox reactions may have serious implications for collection efficiency. For example, the effect of redox reaction(s) will become significant when there are high levels of SO2 in the gas stream. This, of course, occurs in many smelting operations and severely limits the use of both the acidic iodine monochloride method and the acidic permanganate method for the determination of Hg.Evidently no published methods appear where these collection systems have been modified to accommodate high levels of SO2. To make these methods viable, what is needed in the collection system is a scrubbing solution that will remove SO2 and also ensure that the integrity of any Hg trapped in the scrubber is preserved. Removal of the SO2 may be achieved by using either an alkali or a suitable oxidant. However, preservation of the Hg precludes the use of alkali, and in the present work a system utilizing acidic peroxide for SO2 removal and acidic permanganate for mercury collection has been used. Peroxide is an established absorber for SO2 5 and it was felt that the use of dilute acid (0.1N H2SO4), to ensure preservation of the mercury, would not have any adverse effects. With respect to acidic permanganate, only very limited data are available on its collection efficiency for mercury. 4 Field testing of the method was carried out with the objective of (i) evaluating the collection efficiency of acidic permanganate for Hg and (ii) confirming the applicability of using acidic peroxide for the removal of SO2.