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Gas Cleaning System - Arsenic
February 4, 2004
Dry Arsenic Removal
Wet Gas Cleaning Theory
Boliden Wet Gas Cleaning Process
Arsenic can be found in many types of ores. In pyrometallurgical operations, arsenic is easily vapourized and does not present a problem in downstream metallurgical operations. However, arsenic does become a problem in downstream treatment of the off-gases. In the past, arsenic was collected with the other flue dusts and the portion not collected was simply released to the environment. Handling and disposal of the flue dust was also a problem since the arsenic compounds were highly soluble and toxic.
In order to properly deal with the presence of arsenic in the process, the distribution of arsenic in the various process streams must be known. The two major factors affecting arsenic distribution are off-gas temperature and matte grade. Other minor factors are bath temperature, oxygen concentration, mineralogy, and the presence of other impurities.
The majority of arsenic in the off-gas will be present as arsenic trioxide (As2O3) vapour. A small portion will be present in the dust as arsenic compounds. Off-gas treatment involves cooling of the gas which results in the condensation and subsequent removal of As2O3 as a solid. The presence of metals (i.e. iron, copper, lead, zinc, etc.) in the off-gas also contributed to the capture of arsenic due to the formation of metal arsenates.
The traditional method for arsenic removal was to cool the off-gas to condense As2O3 into a solid particle and then collect all the dust in fabric dust collector or bag house. The collected dust was referred to as ‘crude arsenic’ and contained approximately 50 to 85% As2O3 plus other impurities. This method of arsenic removal has for the most part has been replace by more efficient gas cleaning methods. Where the process is used, the bag house would typically be followed by another gas cleaning stage.
It is desirable to operate the bag house as cold a possible (about 120oC) to maximize the amount of arsenic that is condensed and to prevent damage to the fabric filters from excessive temperatures. However, the off-gases may also contained sulphur trioxide (SO3) which in the presence of water will form sulphuric acid (H2SO4). As the gas is cooled, acid will condense when the dew point of the gas is reached. To avoid excessive corrosion of the equipment, the gas temperature is limited to about 10 to 15oC above the dew point of the gas. This obviously limits the amount of arsenic that will condense due to the higher vapour pressure of arsenic at the higher temperature.
Dry collection of arsenic is only possible when the quantity of SO3 in the gas is small. It has been observed that when the ratio of SO3:As2O3 is higher than 1:10, the collected dust on the bags becomes very sticky and difficult to remove.
Arsenic is volatilised in pyrometallurgical operations and is generally present as As2O3 and possibly As4O6 to a lesser degree. Very little arsenic is present as As2O5 despite the high temperatures present in the Fluid Bed Reactor (FBR). At temperature above 300oC, arsenic is present as a metallic vapour or fume. As2O3 will begin condensing at a temperature of 300oC and be essentially completed at a temperature of 200oC. When the FBR gases are quenched to about 70 to 80oC all the arsenic condenses. The quenching of the gas is done in the first vessel of the Quench Tower.
The sudden cooling of the gas to below the dew point of arsenic results in supersaturation of arsenic in the gas and the formation of very fine submicron particles or mist. The effect is similar to what happens when air saturated with water is sudden cooled below its dew point. Also, for mist to form, nuclei must be present in the gas to initiate the process.
From the point of view of designing a gas cleaning system to remove the mist, it is important to know the particle size distribution. Unfortunately, it is impossible to predict the particle size distribution so we must rely on practical experience.
To enable these particles to be removed from the gas in an economical manner requires that the particles grow in size. Particle growth occurs by diffusion of gaseous matter and subsequent condensation on the particle. As well, collision between two cohesive particles will cause the particles to merge together forming a larger one. The movement of the submicron particles in the gas is governed by Brownian particle movement which is the random side-to-side motion observed for small particles and molecules.
The mechanism of particle growth for condensed metallic vapour is primarily Brownian movement. Therefore, particle growth requires only sufficient time in which to allow the particles to collide with each other and increase in size. This retention time is typically provided in the quench vessel itself or a separate retention vessel.
At the exit of the retention vessel, the particles have grown to a size that will permit there removal in the downstream high pressure drop scrubber and wet electrostatic precipitators.
The Boliden Wet Gas Cleaning Process is used at Boliden’s plant at Helsingborg, Sweden for cleaning roaster gases resulting from the roasting of pyrites containing arsenic and mercury.
The process is based on the relatively low solubility of arsenic trioxide (As2O3) in ‘dilute’ sulphuric acid. Solubility curves for As2O3 in sulphuric acid show that the solubility of As2O3 is at a minimum at about 60% H2SO4. Arsenic in acid at this concentration forms crystals that can be separated from the acid.
At the Boliden plant, roaster off-gases pass through a hot electrostatic precipitator to remove the dust in the gas. The gas is then enters an open spray tower where the gas is contacted with a circulating stream of 60% H2SO4. The arsenic is removed from the gas and forms crystals in the acid. The concentration of solids in the circulating acid is controlled by transferring a potion of the acid to a settler or filter.
The gas leaving the first wash tower is sent to a second wash tower and then a gas cooling tower. The weak acid from the gas cooling tower is treated with a solution of sodium sulphide which precipitates mercury and arsenic. The precipitate is separated and transferred to the first washing tower to form a single arsenic containing discharge stream. The arsenic trioxide produced by this gas cleaning process is relatively pure and can be further processed to ‘white’ arsenic.