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Handbook of Sulphuric Acid Manufacturing
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Gas Cleaning System - Mercury Removal
November 16, 2016
Copper, zinc, pyrite and lead ores often contain mercury as a trace contaminant. When these ores are treated in thermal processes, the mercury is volatilized as mercury vapour and exit with the off-gases. Without any special treatment approximately half of the mercury will report to the product acid.
When the ore contains selenium it will also be liberated in the roasting/smelting process along with the mercury. The selenium will be in the amorphous state which will readily combine with mercury to form HgSe. Mercury in this form will be easily removed from the gas in the scrubbing and wet electrostatic precipitation stages.
Mercury will also react with the sulphur dioxide and oxygen in the gas to form mercury sulphate according to the reaction:
Hg(g) + SO2(g) + O2(g) --> HgSO4(s)
Mercury sulphate will be removed from the gas in the subsequent scrubbing and wet electrostatic precipitation stages.
The mercury that remains uncaptured will proceed towards the drying tower as mercury vapour. If left untreated the mercury will enter the strong acid system and contaminate the product acid.
The mercury contaminated acid cannot be used in any industry or products where there is the potential for the mercury to enter the food chain. Therefore, it is desirable to remove the mercury from the acid to produce a more saleable product.
Many different processes have been developed specifically to deal with the mercury problem. The mercury can be treated while it is in the off-gases or after it has entered the product acid.
The removal of mercury as far upstream in the process is desirable in order to minimizes the chance of it entering the final product acid. This involves treating the entire gas volume passing through the gas cleaning section of an acid plant. Various methods have been developed for removing mercury from the gas before it enters the drying tower.
The Outokumpu process is based on converting the elemental mercury in the gas into a sulfate according to the reaction:
Hg + H2SO4 --> 1/2 O2 + HgSO4 + H2O
The smelter gas is scrubbed with 80-90% H2SO4 at a temperature of 150-180°C. The acid is recirculated until the solution becomes saturated with HgSO4 and precipitation begins. The crystals of HgSO4 are then separated in a thickener. In addition to removing mercury, other contaminants in the gas will be removed in the scrubber. The solids collected in the thickener will also contain iron, zinc, copper, selenium, etc.
Mercury can be recovered by mixing the solids with calcium oxide, and then heating to distill away the mercury. Selenium remains behind in the form of calcium selenite.
Mercury is captured in a two drying towers operating in series. The first drying tower operates at an acid concentration of 80% H2SO4 and a temperature below 50°C. The second drying tower is a conventional tower operating at 93% H2SO4. Mercury reacts with the mercury dissolved in the acid to form mercurous sulphate.
HgSO4 + Hg --> Hg2SO4
The mercurous sulphate is then oxidized to mercuric sulphate by the strong acid and oxygen in the gas. A bleed of acid is taken from the drying system to control the concentration of mercuric sulphate in the circulating acid. The acid is treated with sodium thiosulphate to precipitate the mercury as mercuric sulphide. See Sulphide Precipitation - Liquid Phase
This process developed by Boliden is especially suited for low mercury concentrations in the gas. The selenium filter consists of a porous inert material soaked with selenious acid which is then dried to precipitate red amorphous selenium.
H2SeO3 + H2O + 2 SO2 --> Se + 2 H2SO4
The red amorphous selenium reacts with the mercury in the gas to form HgSe. The contact time in the filter is about 1 to 2 seconds. The filter continues to be effective until the level of mercury in the filter reached 10-15%. The filter is then treated to recover the mercury and regenerate the selenium. The filter will remove approximately 90% of the incoming mercury.
Like the selenium filter, this removal process relies on the presence of amorphous elemental selenium to react with the elemental mercury in the gas. Sulphuric acid containing selenium is circulated over a packed tower . The acid concentration is maintained between 20 and 40%. At lower concentrations selenium will form highly soluble selenium-sulphur compounds making it ineffective in reacting with the mercury in the gas. At higher acid concentrations, the oxidizing power of the acid will result in selenium dioxide or selenite being formed.
If the gas being treated contains sufficient selenium, there may not be a requirement to add selenium to the scrubber solution.
The selenium scrubber is suitable for removing relative large quantities of mercury in the gas and has an efficiency of approximately 90%.
The Boliden Norzink process was developed in 1972 and is the most popular method for removing mercury from the process gas. The process is based on the oxidation of mercury vapour by mercuric chloride to form mercurous chloride (calomel).
HgCl2 + Hg --> Hg2Cl2
A solution of mercuric chloride containing xx mg/l HgCl2 is circulated over a packed tower. The process gas containing mercury passes through the packing where the mercury reacts with mercuric chloride to form mercurous chloride. Mercurous chloride is insoluble and precipitates out of the solution.
A side stream from the main tower circulating stream is directed to the primary settler where the mercurous chloride settles to the bottom of a conical tank. The clarified solution overflows back to the scrubbing tower pump tank. The collected solids from the bottom of the primary settler flows to the secondary settler where further concentration of the mercurous chloride. In the secondary settler, zinc dust can be added to further aid in the precipitation of mercury from the solution. The solids from the secondary settler are discharged to storage drums for sale or further processing.
The scrubbing process removes mercuric chloride from the scrubbing solution. If the concentration of mercuric chloride is not maintained it will become ineffective as a scrubbing solution. To regenerate the scrubbing solution, a portion of the mercurous chloride collected is chlorinated to regenerate the scrubbing solution. A separate tank and circulating system is used to regenerate the scrubbing solution. Chlorine is injected into a circulating stream which reacts with mercurous chloride to form mercuric chloride.
Hg2Cl2 + Cl2 --> 2 HgCl2
When the solution is completely regenerated it is pumped into a storage tank. As the concentration of mercuric chloride in the main scrubbing circuit is depleted, make-up solution is taken from the storage tank to maintain the concentration of mercuric chloride in the solution.
This method is very effective in removing mercury from the gas. A product acid containing less than 0.5 ppm mercury can be produced from a gas containing 150 ppm mercury.
The mercury absorption tower is a fibreglass reinforced vertical cylindrical vessel. The tower is packed with polypropylene packing, generally saddles. The scrubbing solution is sprayed onto the top of the packing through a series of nozzles. A chevron and mesh pad mist eliminator at the top of the tower prevents the carryover of scrubbing solution out of the scrubber.
Elemental mercury can be recovered by electrowinning. All the mercurous chloride formed is reacted with chlorine to form a solution of mercuric chloride. Electrowinning the solution of mercuric chloride in a specially design cell using metallic mercury as the cathode will produce elemental mercury and chlorine gas at the anode.
Overall Reaction HgCl2 + e = Hg + Cl2
Cathode Reaction Hg2+ + 2e = Hg
Anode Reaction 2 Cl- = Cl2 + 2e
Under unfavourable conditions hydrogen gas can be formed at the cathode. At the anode the formation of oxygen is a competing reaction.
The chlorine that is produced can be reused in the chlorination reaction that converts mercurous chloride to mercuric chloride. If no chlorine is loss in the process there will be no requirement for fresh chlorine gas since the overall process generates as mush chlorine as it consumes.
The electrolyte solution is continuously recirculated over the cell to avoid the formation of concentration gradient in the cell. The cell is kept at a slight vacuum in order to prevent the leakage of chlorine gas out to the environment.
The technology developed by Boliden Norzink was purchased by Outokumpu in 200x and is now part of Outotec which was spun off from Outokumpu Technology.
Activated carbon is well known for its adsorption properties. For the adsorption of mercury, activated carbon can normally adsorb 10-12% of its own weight. The operating temperature of the carbon filter is limited to 50°C. The method is especially suitable for low mercury concentrations in the gas. A 90% removal efficiency is normally achievable.
Sulphide Precipitation - Gas Phase
A controlled amount of hydrogen sulphide gas is injected into the gas phase which reacts with mercury to form mercury sulphide (HgS).
2 H2S + SO2 --> 3 S + 2 H2O
S + Hg --> HgS
H2S + Hg --> HgS + H2
The mercury sulphide that is produce is scrubbed out of the gas in downstream gas cleaning equipment and reports to the weak acid effluent stream. The consumption of hydrogen sulphide is approximately 1.5-2.0 times the stoichiometric amount. Approximately 90% of the mercury in the gas is removed.
This process has successfully been used by St. Joe Minerals Corporation at their zinc smelter in Monaca, Pennsylvania, USA.
Sumitomo Metal Mining Engineering Co., Ltd. (SMMEC) process is based on the absorption of mercury onto coke and a metal sulphide material like copper concentrate. Mercury laden gas passes through a bed of coke and copper concentrate where it is removed from the gas by absorption.
A 1500 mm thick bed can give a 95% removal efficiency at a bed velocity of 0.2 m/s. The bed has a low pressure drop of 0.2 kPa/m at a gas velocity of 0.2 m/s.
The mercury absorber would be located after the wet ESP and before the Drying Tower.
If mercury is not removed from the gas stream before entering the drying tower it will report to the product acid. Treating the product acid will generally involve smaller process equipment since you are dealing with a liquid stream instead of extremely large gas volumes. The disadvantage is that it is generally desirable to remove contaminants as far upstream in the process as possible.
Sulphide Precipitation - Liquid Phase
Colloidal sulphur can be created in the acid by the addition of sodium thiosulphate. The reaction to form colloidal sulphur is as follows:
H2SO4 + Na2S2O3 -> S + Na2SO4 + H2O
The sulphur will react with the mercury to form crystalline mercury sulphide (HgS). Other metal contaminants in the acid will also react with the sulphur to form insoluble metal sapphires.
The method is restricted to acid concentrations less than 85% H2SO4. At higher acid concentrations the sulphur is oxidized to sulphur dioxide (SO2). As well, the product acid will also contain sodium sulphate which may be undesirable in the product acid. The proper dosage of sodium thiosulphate is also required otherwise a very fine mercury sulphide is produced which is difficult to filter. This method is capable of reducing the mercury levels from 15 ppm to 0.5 ppm after a holding time of 1 hour.
Hydrogen sulphide could also use as the source of sulphide for precipitation of mercury and other metals. This method is preferable is the presence of sodium sulphate is undesirable in the final product.
