Scrubbing processes mercury removal

Zeolites have also proven applicable for removal of nitrogen oxides (NO ) from wet nitric acid plant tail gas (59) by the UOP PURASIV N process (54). The removal of NO from flue gases can also be accomplished by adsorption. The Unitaka process utilizes activated carbon with a catalyst for reaction of NO, with ammonia, and activated carbon has been used to convert NO to N02, which is removed by scrubbing (58). Mercury is another pollutant that can be removed and recovered by TSA. Activated carbon impregnated with elemental sulfur is effective for removing Hg vapor from air and other gas streams the Hg can be recovered by ex situ thermal oxidation in a retort (60). The UOP PURASIV Hg process recovers Hg from clilor-alkali plant vent streams using more conventional TSA regeneration (54). Mordenite and clinoptilolite zeolites are used to remove HQ from Q2, clilorinated hydrocarbons, and reformer catalyst gas streams (61). Activated aluminas are also used for such applications, and for the adsorption of fluorine and boron—fluorine compounds from alkylation (qv) processes (50). 

Mercury and cadmium are more volatile (boiling points 356 and 765 °C respectively) and are consequently more difficult to remove. They are reduced to about 0.2mg/Nm in total in the semi-dry scrubbing and pre-cooled dry injection processes and to about 0.1 mg/Nm in the wet scrubbing processes [29.10]. Where lower emission levels are required, the injection of activated carbon can reduce the level to 0.05 mg/Nm [29.10]. 

As previously mentioned, all impurities besides mercury are removed by a first washing and cooling step in the acid plant. The mercury goes throu this wash step and requires a separate removal stage. Three different scrubbing processes are available for the removal of metallic mercury from the SO gas. Processes for removal from the acid are also used, but not frequently in the electrolytic zinc process. All processes produce a sludge that is either stored or used for the production of metalHc mercury. 

Boliden-Norzink process". This removal step is performed after the washing and cooling step in the acid plant hence the gas is dust and SO, free and the temperature is about 30 C. The gas is scrubbed in a packed bed tower with a solution containing mercuric chloride HgCl. This reacts with the vapor of metalHc mercury in the gas and precipitates it as calomel, Hg Cl ... 

Following scrubbing and cooling, suspended droplets of scrubbing solution or mist are removed in wet electrostatic precipitators. The cleaned gas may stiU contain unacceptable levels of mercury vapour, which must be removed before transfer of gas to the acid conversion plant. There are a nnmber of mercnry removal techniques but the most commonly used is the Boliden-Norzink mercury removal process, which uses mercuric chloride solution to scrub the gas and absorb mercury according to Eqnation 4.7 (Dyvik, 1985) ... 

One such process was developed by the Outokumpu Company in Finland and used in the company s zinc smelter at Kokkola. The gases are scrubbed with concentrated sulfuric acid, whereby mercury is dissolved as mercuric sulfate. After several recycling operations, the mercury concentration reaches saturation level such that crystals of mercuric sulfate form and can be removed. 

Lead ores are contaminated with mercury which is poisonous to a vanadium catalyst. Hence, it is first removed by the patented Norwar process by scrubbing with mercuric chloride and recovering mercury by chlorination ... 

Section 9.2.5.1 also mentions the removal of mercury from hydrogen by scrubbing with chlorinated brine or water. The same process can be used to scrub air, as from cell end boxes, when enough differeutial pressure is available to operate a column with some degree of efficiency. The chlorine in the scrubbing liquor oxidizes the mercury, which... 

Usually, natural gas treatment on the basis of thermal process engineering takes place in three steps (see Fig. 7.9). The first step that may consist of partial steps just like all other subsequent steps, serves the preparation of the crude gas for its processing. Here, for example, acid-forming gas components, such CO2, H2S and other sulphuric compounds are removed. Usually, chemical scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent is being regenerated. Then the natural gas is dried. In case of moderate water dew point requirements, glycol is used as wash liquor. The lowest water contents (< 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally, mercury is removed in case aluminium will be used as material of construction for equipment. Mercury in contact with aluminium may lead to catastrophic corrosion. 

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