Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/69—Sulfur trioxide; Sulfuric acid
  • C01B17/90—Separation; Purification
  • C01B17/906—Removal of mercury
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/48—Sulfur dioxide; Sulfurous acid
  • C01B17/50—Preparation of sulfur dioxide
  • C01B17/56—Separation; Purification
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/69—Sulfur trioxide; Sulfuric acid
  • C01B17/74—Preparation
  • C01B17/76—Preparation by contact processes
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G13/00—Compounds of mercury

Definitions

• the present invention relates to a method for producing pure sulphuric acid and a high-grade mercury product in the manufacture of sulphuric acid from a substantially dry gas which contains sulphur- dioxide and which is obtained when treating mercury-containing raw materials and intermediate products, 'the gas being cleansed of gaseous mercury accompanying said gas, by contacting the gas with a sulphuric-acid solution in one or more stages, so that the major part of the mercury is taken-up by and dissolved in a concentrated solution with a sulphuric-acid concentration of at least about 90%.
• mercury-containing raw materials is meant primarily metal sulphides, such as pyrite, chalcopyrite, zinc-blende and cinnabar, whi le by intermediate products is meant primarily mercury-containing sludge, deriving, for example, from wet-gas and dry-gas cleansing processes.
• the sulphur-dioxide-containing gases referred to are primari ly roaster gases obtained when roasting sulphidic minerals, although gases obtained when working-up intermediate products that contain mercury, may also form part of the sulphur-dioxide- containing gas.
• O PI Sulphuric-acid which is contaminated with mercury can also be obtained from sulphuric-acid manufacturing plants based on the roasting of metal sulphides, for example, py ⁇ ' te, or zinc-blende, when the roaster gases are not cleansed thoroughly enough. Even though the gas is cleansed adequately, breakdowns in the operation of such sulphuric-acid plants can, at times, result in gases which are highly contaminated with mercury.
• metal sulphides for example, py ⁇ ' te, or zinc-blende
• the roaster gases generated when roasting sulphide materials are passed from the roaster to, for example, a cyclone, where the gases are cleansed from coarse dust particles accompanying said gases.
• the gases are then cooled and dry-cleansed in , for example, a Cottreli precipitator.
• Final cleansing of the gas is effected, for example, by washing the gas in washing towers with subsequent wet Cottreli filters.
• There is normally no difficulty in expelling mercury compounds, and consequently the major part of the mercury present in the material will be incorporated in the roaster gas in the form of mercury compounds and elementary mercury in particle or vapour form. Substantially all the mercury compounds present in the gas can be separated therefrom in particle form , by means of such gas- cleansing systems.
• a purifying method for this purpose requi res firstly that low residual contents of mercury can be achieved in the sulphuric acid and secondly that the precipitated mercury-containing material can be removed from the acid.
• it is not permitted for other toxic substances, such as lead and arsenic, to be introduced into the acid.
• fast acting processes are required, for example so as to limit the corrosion attack on apparatus contained in the purifying plant when handling dilute acids and washing liquors.
• German Patent Specification No. DE-C-I 216 263 describes a method in which concentrated sulphuric acid is treated with relatively coarse particulate elementary sulphur.
• the residual contents obtained are not acceptable in view of the fact that many countries have stringent requi rements with respect to the deposition of heavy metals.
• Mercury can also be precipitated from sulphuric acid, di luted or concentrated, by treating the acid with sulphides or hydrogen sulphides.
• the disadvantages with these methods described for example in the German Patent Specifications, DE-C-I 054 972 and I 124 024, are that the acid can be contaminated by metals introduced as metal sulphides, and that it is difficult to separate the precipitated mercury compounds from the acid. Account must also be taken of the hygiene risks created when producing or using hydrogen sulphide.
• Mercury is precipitated from the sulphuric acid very rapidly when , in accordance with SE-B-369295, elementary finely -divided sulphur is precipitated in the sulphuric acid, by adding thereto a sulphur compound, which decomposes in the acid to form colloidal sulphur, on which mercury present in the acid is adsorbed.
• the sulphur compound may comprise H ⁇ S or Na-S, although thiosulphate is preferred.
• the residual mercury -content of the acid is extremely low. This separation , however, is encumbered with some difficulties, due to the large specific surface area of the sulphur.
• the filtering process is extremely time-consuming.
• the sulphuric-acid purification process combined with the washing system is also encumbered with the afore-described disadvantages, concerning the introduction of other impurities into the acid. Difficult problems in separating the mercury from the acid are also experienced.
• roaster gases in two (or more) stages at different acid concentrations is also described in DE-B-I 792573 in conjunction with a method for avoiding the manufacture of 'black ' acid from roaster gases that contain organic compounds.
• none of the purifying methods mentioned above fulfills the requirements mentioned in the introduction with respect to low residual contents of mercury and other metals in the sulphuric acid produced, and that it must be possible to isolate effectively the mercury compounds removed.
• the best methods from the aspect of purifying sulphuric acid namely those methods based on forming a " finely-divided sulphur-phase in the sulphuric acid in order to adsorb the mercury, are encumbered with difficult separation problems.
• this means that the possibility of treating materials which are more rich in mercury than others is restricted, since wide variations in the ingoing mercury content cannot be tolerated unless the purifying apparatus has been initially over-dimensioned.
• all of the mercury product recovered will be of a low grade , since during the purifying process other impurities in the acid, among others, selenium , which is often present in the acid, will also be adsorbed at the same time on the precipitated sulphur-phase, thereby to contaminate the sulphur itself and the added filter aid, and to substantially 'dilute' the separated mercury product.
• selenium which is often present in the acid
• the precipitated sulphur-phase thereby to contaminate the sulphur itself and the added filter aid, and to substantially 'dilute' the separated mercury product.
• there are to be found large quantities of such low-grade mercury-containing products obtained, inter alia, from gas-cleansing filters and washing apparatus, which cannot be worked-up economical ly, and which merely constitutes a dumping problem and consequently also a potential threat to the environment.
• the present invention is based on absorbing mercury in sulphuric acid of high concentration and is characterised by the steps set forth in the following claims.
• the major part of the elementary mercury accompanying the substantially dry gas is absorbed in sulphuric acid having a concentration of at least 90%, from which acid mercury is recovered, by contacting the acid with a powerful oxidizing agent such as sulphur trioxide or oleum , so as to oxidize mercury present in the acid, whereupon mercury dissolved in the acid is oxidized to divalent mercury.
• a powerful oxidizing agent such as sulphur trioxide or oleum
• Fig. I is a diagram i l lustrating the solubi lity of mercury • and Fig. 2 is a process schematic illustrating a preferred embodiment of the invention.
• substantially dry gas is not necessarily meant that the water content of the gas is zero or close to zero. It is sufficient for the gas to have been dried to a water content corresponding to that obtained when drying the gas with sulphuric acid having a concentration of about 70% and thereabove. Residual moisture in gas which has been pre-dried in this manner, or which previously contains correspondingly low moisture contents, will not deleteriously affect the water balance in the final -drying circuit, which contains concentrated sulphuric acid. On the other hand, however, serious problems will occur with the water balance in the final-drying circuit if the acid has a higher moisture content.
• Figure I illustrates how the solubility of Hg and Hg in sulphuric acid vary with the concentration of the acid.
• the solubility of elementary mercury (Hg ) has also been plotted on the diagram , for the sake of comparison.
• the curves shown refer to the solubility at room temperature.
• the solubility of Hg decreases with increasing acid concentrations, but exhibits a minimum between 80 and 90% and lies about 30-50 g/t at relevant concentrations between 70 and 100%.
• the concentrated acid contains a level of both Hg + and Hg + which lies close to the saturation limit
• Hg wi ll be formed in quantities above the saturation limit, and consequently mercury (ll)compounds will precipitate, while the content of mercury (I) ions will fall far beneath the saturation limit.
• the acid is better able to absorb mercury dissolved , as Hg + in the drying tower where the acid is brought into contact with the mercury containing gas.
• Oxidation of the sulphuric acid can be effected in a separate stage , with an oxidant which is sufficiently powerful to oxidize the mercury-content of the acid, or a substantial part of said mercury content, to a divalent state.
• an oxidant which is sufficiently powerful to oxidize the mercury-content of the acid, or a substantial part of said mercury content, to a divalent state.
• powerful oxidants include halogens, hydrogen peroxide , oxygen, ozone and ammonium peroxodisulphate.
• oxidation is effected while maintaining
• CMP the concentration of the acid in a separate absorption tower connected to the circuit and by charging sulphur trioxide or oleum to the acid in said tower, to increase the concentration of the acid to the desired high level.
• sulphur trioxide sulphur dioxide dissolved in the acid wi ll be effectively removed and isolated at the same time , and passed to the contact apparatus instead.
• the method according to the invention can be carried out in a number of different ways within the scope of the claims, the preferred method in each individual case being chosen with respect to the apparatus available and to any previous purifying methods which may already have been practised , on site, for eliminating mercury.
• the method according to the invention can advantageously be applied in combination with a single-stage sulphuric-acid washing process, for example of the kind described in DE ,B-213223I , and wi ll then positively affect the mercury-absorption of the wash and provide a purer mercury product.
• a plant for drying and purifying roaster gases in a double-dryer system comprises a pre-drying circuit I an after drying circuit 2, an absorption ci rcuit 3, and a stripper 12.
• the reference 4 identifies a converter for oxidizing sulphur dioxide to sulphur trioxide in accordance with the contact method, said converter being associated with a sulphuric-acid manufacturing plant.
• the circuits 1-3 enable acid to be cycled internally, within the drying towers incorporated in respective circuits. This is not shown in the Figure, since otherwise the process schematic could not be readily followed.
• Each circuit is suitably provided with droplet separators (not shown) , arranged to prevent acid droplets from accompanying the gas in the process, and to the converter 4.
• a moist roaster gas which contains sulphur dioxide and also elementary mercury is passed to the pre-drying circuit I , through the line 5.
• the roaster gas obtained from the roasting process contains solid mercury compounds, these are effectively captured, for example , in the water wash effected in a washing tower (not shown) , which precedes the drying apparatus and in which the gas is saturated with water.
• the gas is dried in the pre-drying circuit I with sulphuric acid, which is supplied to the drying circuit 1 from an absorption circuit 3 or an after-drying circuit 2, through the line 6.
• the gas is contacted with the sulphuric acid in counter-flow and the major part of the water contained by the gas is taken-up in the acid.
• OIKPI elementary mercury accompanying the gas is taken-up in the acid in said circuit.
• the acid is passed from the pre-drying ci rcuit I , through lines 8 and 9 , to a stripper 12, in which the acid is blown clean from sulphur-dioxide accompanying the acid, and dissolved therein , by means of air, which is introduced into the lower part of the stripper 12, as indicated by the arrow II , whereupon ai r and stripped sulphur-dioxide is supplied to the gas from the pre-drying ci rcuit I , through a line 4IA. Additional acid can be supplied to the ci rcuit I from the final-drying ci rcuit through lines not shown.
• the acid is passed to a sludge separator 14 through a line 13.
• Separated mercury compounds , primari ly mercury ( I) sulphate present in the acid are therewith drawn-off , in the form of a sludge having a high mercury content , through a line 16.
• the sludge drawn-off through the line 16 can be readi ly worked-up to elementary mercury or other commercial ly valuable compounds.
• the acid which has been freed from mercury sludge is passed through a line 15 to a separate sulphuric-acid purifying plant , in which the acid is high ly purified in a suitable manner, for example , in the manner described in our earl ier Patent Specifications SE-B- 369295 and 7307048-4.
• the gas is passed from the pre-drying ci rcuit I through line 41 to an after-drying ci rcuit 2 , and from there to the converter 4, through line 42, and from the converter through line 43 ,44 to an absorption tower (not shown) belonging to the associated sulphuric-acid plant.
• Acid having a concentration of 96-98.5% is passed through a line 17 from the absorption tower of the sulphuric-acid plant to the after-drying circuit 2. Acid is also charged to the circuit 2 via an external ci rculation ci rcuit for concentrated acid.
• This ci rculation ci rcuit comprises a line 18, a fi rst sludge separator 19, a l ine 20 , the absorption ci rcuit 3, a line 21 , a second sludge separator 19A , and a line 22.
• the acid is cleansed of mercury compounds
• the sulphur trioxide supplied wi ll be dissolved in the acid passing through the circuit, thereby enabling the concentration of said acid to be maintained at the desired high level.
• This supply of strongly oxidizing sulphur-trioxide to said acid also results in the oxidation of mercury present in the acid to a divalent state, so as to enable a mercury (II) ion content to be achieved up to the saturation level. When this content reaches a level above the saturation level , mercury salts will precipitate.
• the after-drying ci rcuit 2 may incorporate one or more drying towers, either arranged separately in series, or directly connected to one another, for example arranged one above the other as two or
• the acid concentration is suitably selected at about 90-95% in the first tower, and 95-99% in the second tower, which is located nearest to or immediately prior to the contact plant.
• oleum can be supplied to the after-drying ci rcuit from a separate oleum-production tower.
• Sludge separated in the sludge separators 19 and I9A is passed through lines 23 and 23A, to de-aerator 24, to which air is passed as indicated by the arrow 25.
• sludge that contains mercury can be drawn-off from the de-aerator 24, through a line 25, although in the majority of cases the sludge downstream of the de-aerator is passed through a line 26 and supplied to the sludge separator 26, and there mixed with the sludge arriving from the pre-drying circuit I , to a sludge-mix from the circuit, through line 16.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Treating Waste Gases (AREA)

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Publications (1)

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Family Applications (1)

Country Status (5)

Citations (4)

• 1982
  • 1982-12-03 SE SE8206918A patent/SE439154B/sv not_active IP Right Cessation
• 1983
  • 1983-12-01 WO PCT/SE1983/000419 patent/WO1984002122A1/en not_active Application Discontinuation
  • 1983-12-01 JP JP84500151A patent/JPS59502145A/ja active Pending
  • 1983-12-01 EP EP19840900124 patent/EP0127664A1/en not_active Withdrawn
  • 1983-12-01 AU AU23377/84A patent/AU2337784A/en not_active Abandoned

Patent Citations (4)

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