Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1999-04-14
2001-09-04
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S243100, C423S243080, C423S243110, C423S244070, C423S244080
Reexamination Certificate
active
06284208
ABSTRACT:
The present invention refers to an improved process and device for the removal of mercury and sulphur dioxide from flue gases from, for example, smelters, power stations, waste incineration plants, and crematories, by washing the gas with chloride containing water, such as sea water, to which is added an oxidizing agent which oxidizes all of the mercury to Hg(II) ions, and by using such high redox value in the washing water that Hg
+
salts cannot precipitate. The gas and the washing water are passed co-currently down through a bed of consumable limestone. The mercury(II) ions will then dissolve easily in the chloride containing washing water, as a chloride complex, and the oxidized sulphur oxide is converted to sulphuric acid being neutralized by the consumable limestone, such that the pH value of the washing water remains almost constant without the need for using an aqueous solution of sodium hydroxide as a neutralizing agent. Further, the bed of limestone will be functioning as a turbulent mixer of gas and washing water for a rapid oxidation, and since the limestone is slowly consumed, deposits such as gypsum and dust will continuously be washed away from the limestone gravels without clogging the bed.
The present process is carried out by adding to the gas washing water containing 30-40 g chlorides/liter, an oxidizing agent, such as sodium hypochlorite, until the redox potential of the water is within the range 600-800 mV. The gas washing water and the flue gas are fed co-currently down through the lime stone bed, in which they are mixed intimately for a rapid reaction and absorption of the mercury in the aqueous phase.
As flue gases also often contain SO
2
gases, sulphuric acid is formed by the oxidation which reduces the pH value of the gas washing liquid.
According to the present invention, the gas is passed co-currently with the gas washing water through the bed of, for example, limestone. The limestone will neutralize the acid(s), and hence the pH of the gas washing water will remain nearly constant in the range 5-6 pH units, and thus completely eliminate the need for the use of aqueous sodium hydroxide, as well as the possibility of SO
2
reducing the oxidized mercury to Hg
+
salts which can precipitate, or to metallic Hg which is reintroduced into the flue gas.
The bed of consumable limestone is constantly washed with the gas washing liquid, so that dust and formed sulfates, such as gypsum and other particles, are continuously washed into a collection tank arranged below the gas washing tower, from which the washing water is conducted to an adjacent water purification plant in which the mercury is removed as sulfide by common precipitation.
Gypsum particles and other particles act as an in situ filter aid for the minute precipitated mercury sulphide particles in the water purification plant, thus easing the separation of slime and water, for instance by filtration, and the chloride water, which will be practically devoid of mercury (<1 &mgr;g Hg/1), can either be disposed to a recipient or returned to the gas washer.
In addition, it is preferred to conduct the gas and the gas washing water co-currently down through the bed of lime-stone and into the collection tank for the gas washing water, in which tank the gas washing water and the gas phase meet the surface of the water in the tank. The water droplets in the gas phase will then be absorbed on the water surface by adjusting the distance between the bed of limestone and the water surface in the collection tank for the gas washing water as well as the length of said surface, before purified gas is exhausted to a chimney. This will eliminate the need for complicated droplet-catchers, being in common use in counter current gas washing.
It is known that the mercury in the flue gases exists both as oxidized mercury and metallic mercury vapour. The metallic mercury vapour is the most difficult to remove with wet washing of the flue gas, as the vapour must be oxidized before it can be absorbed in the gas washing water.
Flue gases from smelters and power stations have a relatively even content of mercury and other gases, such as SO
2
, which reduces the mercury to metallic vapour in the flue gas, which can contain from 10-20 &mgr;g Hg/Nm
3
up to 300-500 &mgr;g Hg/Nm
3
.
The smoke gases from waste incineration plants and crematories show large variations in the consentration of mercury. The consentrations can be 100-10.000 &mgr;g/Nm
3
gas over a short period of time, 5-10 minutes, due to sources containing large amounts of mercury, such as dental amalgam.
A wet wash process for effective removal of mercury hence must have very quick reaction kinetics to be able to handle these relatively short and intense peaks of mercury in the flue gases.
In accordance with the invention, it is preferred to add sodium hypochlorite to a gas washing water based on sea water, such that the redox potential of the sea water remains in the range 600-800 mV, and subsequently feed the gas and the gas washing water down through a bed of limestone, such that the pH in the gas washing water stays between 4 and 6 pH units, and subsequently into a tank in which particles and the gas washing water are collected, whereafter the gas washing water is purified in an adjacent water purification plant, and one part of the purified water is returned to the gas scrubber and one part is led to a recipient. This process removed 90-93% of the mercury from the flue gas from a smelter at a residence time in the gas scrubber of only 0.05-0.10 seconds.
The purified flue gas contains 11-16 &mgr;g Hg/Nm
3
after purification.
The present invention is a simplification and improvement of known methods, which is in demand with the increasing concern about mercury pollution, in particular from waste incineration plants and crematories, but also from smelters and power stations.
Oxidation of mercury with absorption in a chloride containing gas scrubber water has been described in numerous publications and patents, inter alia U.S. Pat. No. 5,009,871.
U.S. Pat. No. 5,009,871 teaches that reducing gases such as SO
2
gas reduces oxidized Hg to metallic Hg, which leaves the gas washing water and is reintroduced into the flue gases. In order to prevent this, U.S. Pat. No. 5,009,871 teaches that the redox in the gas washing water must be >300 mV, and preferably in the range 530-710 mV for approximately 80% reduction of the Hg contents in the flue gases.
Further, U.S. Pat. No. 5,009,871 teaches that due to hydrochloric acid and other reducing gases in the flue gas from incinerators, the addition of, for example, NaOH to the gas washing water is necessary in order to give a pH of 8.
It has surprisingly been found that the teaching of U.S. Pat. No. 5,009,871 concerning the addition of caustic (NaOH) to increase the pH to approximately 8 in the gas washing water is not necessary when the gas washing water an the gas is led co-currently down through a bed of limestone.
A person of ordinary skill in the art will know that at pH values of 8 in the gas washing water, carbonates are formed when CO
2
is present in the gases. The produced carbonate can easily be deposited on the surface of the gas scrubber, in valves, tubes, and in tanks, and hence a pH of 8 in the gas washing water may be undesirable when the gas contains CO
2
gas.
Carbonate formation decreases at pH values of <6 in the gas washing water and is completely eliminated at pH <4.
In accordance with the present invention, the pH in the gas washing water consisting of sea water is always in the range 5-6 units, and no formation of carbonates was observed, whereas it was observed when NaOH was used at pH values of 8.
This could in no way be deduced from U.S. Pat. No, 5,009,871, which describes an apparatus which bubbles a gas through an aqueous solution at the rate of 1 l/min (column 8, lines 1-10).
U.S. Pat. No. 3,849,267 teaches that if chlorine gas is added to a gas containing mercury before the gas mixture is fed to beds consisting of, respectively, a 30 cm thick bed of 2 inch saddle formed fill
Griffin Steven P.
Ladas & Parry
Vanoy Timothy C.
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