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
  • C01B17/48—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
  • 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
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G13/00—Compounds of mercury
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B43/00—Obtaining 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 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 ta en-up by and dissolved in a concentrated solution with a sulphuric-acid concentration of at least about 85%.
• mercury-containing raw materials is meant primarily metal sulphides, such as pyrite, chalcopyrite, zinc -blende and cinnabar, while 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 primarily 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.
• 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, pyrite, 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, pyrite, 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 Cottrell precipitator.
• Final cleansing of the gas is effected, for example, by washing the gas in washing towers with subsequent wet- Co tt re 11 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 requires 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 2I6263 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 requirements with respect to the deposition of heavy metals.
• Mercury can also be precipitated from sulphuric acid, diluted 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 054972 and
• 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. Although other separation methods have been proposed, none of these has been able to compete with the filtering method, despite the problems associated with such methods.
• 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.
• the treatment of roaster gases in two (or more) stages at different acid concentrations, as employed in the previous method, is also described in DE-B-I 792 573 in conjunction with a method for avoiding the manufacture of 'black* acid from roaster gases that contain organic compounds.
• OMP ⁇ . W1PO , ⁇ -> 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 present invention is based on absorbing mercury in sulphuric acid of high concentration in one or more stages, and is characterised by the steps set forth in the accompanying claims.
• the method involves absorbing the major part of the elementary mercury accompanying the gas in sulphuric acid having a concentration of at least 85%, from which mercury is recovered by diluting the concentrated acid to a given level at which mercury-ions present in the acid have a low solubility, in this way, there is quickly obtained a high-grade mercury precipitate which can readily be separated from the acid and which can readily be worked-up into a commercially valuable product, such as metallic mercury for example.
• This also greatly facilitates any final purification to which the acid might be subjected, since the major part of the mercury has been removed from the acid in earlier stages.
• Figure I is a diagram illustrating the dilution requirement
• Figure 2 is a block schematic of a preferred embodiment of the invention
• Figures 3 and 4 are diagrams illustrating the purification effect obtained with different acid concentrations and acid temperatures respectively.
• the acid is diluted so as to obtain a concentration less than at least 80%. Even though in certain cases concentrations below 50% may be desirable, it is naturally preferred to dilute as little as possible, and the optimal range is thus from 50-75%.
• a sulphuric-acid-purifying plant is avai lable, however, or if it is planned to incorporate such a plant in a new complex, dilution is suitably halted when reaching a given content of between 50 and 500 g/t, whereafter the mercury precipitate is separated from the solution.
• this mercury range corresponds to a sulphuric-acid concentration lying within the range of 65-75%.
• this quantity often constituting an insignificant percentage of the original mercury content of the acid, and can readily be removed by means of other suitable methods, for example, by precipitation with thiosulphate.
• the thiosulphate is best added to the solution subsequent to diluting the acid to at least concentrations in the range of 70-85% HpSO .. It is also possible, and particularly suitable to purify the sulphuric acid together with other sulphuric acid containing moderate quantities of mercury and having approximately the same concentration range as, for example, sulphuric acid obtained from pre-drying stages in a multi-stage system, as described in more detail hereinafter. As before indicated, it is naturally also possible to utilise dilution precipitation according to the invention while adding sulphur dioxide for the purpose of further purifying the acid.
• the method according to the invention can be carried out in many different ways within the scope of the claims, the preferred method being selected, in each individual case, with respect to the apparatus available, and with respect to any previous purifying processes which may have been carried out for the purpose of eliminating mercury from the sulphuric acid.
• the method according to the invention can be advantageously applied in combination with a single-stage sulphuric-acid washing process, for example, of the kind described in DE-B-2 I32 23I, and will then positively influence the mercury absorption in the washing step, and provide a purer mercury product.
• the method can be applied to particular advantage in multi-stage, gas-cleansing and sulphuric-acid purifying plants of the so-called double-dryer kind, for example, in such systems as those described in SE-B-7 307 048-4 and DE-A-2 243 577, it being possible to further improve these plants and render them more effective.
• This also enhances the possibilities of working-up material that is rich in mercury, for example, mercury-rich sludge, obtained from gas-cleansing systems, since the load on the acid-purifying section of said systems will be greatly reduced.
• precipitating reagents such as thiosulphate , for example and less filtering problems, thereby avoiding bottle-necks in the purifying systems.
• FIG. 1 is a diagram illustrating the extent to which dilution is necessary in the case of a 98%-sulphuric acid as a function of the desired sulphuric-acid concentration in percent by weight.
• the diagram shows that dilution from 98% to about 65%, which lies well beneath the upper limit for dilution according to the main claim , requires the addition of only 0.5 ton of water per ton of acid.
• the water is added to a part-flow of the concentrated acid, this part -flow corresponding approximately to only 0.5-2% of the total acid produced, and hence the total amount of water added is actually quite moderate.
• a plant for producing sulphuric acid in accordance with the contact method comprises a pre-drying tower I , and an after-drying tower 2, a contact apparatus 3, and an absorption tower 4.
• a moist roaster gas containing sulphur dioxide and also elementary mercury is supplied to the pre-drying tower I , via a line 5.
• the roaster gas obtained from the roasting process contains solid mercury compounds, these are effectively captured, for example, in the water wash, which takes place in a washing tower (not shown), preceding the drying apparatus, where the gas is saturated with water.
• the gas is dried in the pre-drying tower I , with sulphuric acid, in a closed circuit, the sulphuric acid being supplied to the drying-tower I, through lines 14 and 7.
• the sulphuric acid contacts the gas in the tower I in counter-flow, and the major part of the water contained by the gas is taken-up in the acid. This lowers the concentration of the acid. The concentration, however, should not be permitted to fall below 70-85%, since the vapour pressure above the acid would be much too high to obtain effective drying of the gas. About 10% of the elementary mercury accompanying the gas is absorbed in the acid in this circuit.
• the gas is passed from the pre-drying tower I, to the after-drying tower 2, through the line 41 , and is passed from said after-drying tower through the line 42, to the contact apparatus 3, and from there through the line 43, to the absorbtion tower 4, and finally in the form of residual gases to the chimney, through line 44.
• the acid is passed from the pre-drying tower I , through the line 8 to the separation tank 9, and from there to an acid cooler II , by means of a circulation pump I0, where the acid is cooled indirectly with water to a temperature of 20-IOO°C, preferably 30-80°C, the water being supplied through line 12, and removed through line 13. Additional acid can be supplied to the tower I, through line 6.
• This acid may comprise acid obtained from external sources or acid n
• the acid is returned to the pre-drying tower through line 14 and 7.
• Chilled acid is tapped-off through line 15, and passed to a mixing tank 16, where a thiosulphate solution is supplied to said acid through line 17, and optionally also water through line 18, in order to improve the precipitation of mercury.
• the sulphuric acid is passed from the mixing tank 16, through line 19, to a separation means 20, where precipitated solid material containing the mercury content of the sulphuric acid is separated out.
• the separation means 20 may comprise a filter press , a centrifuge fi lter or some other suitable apparatus capable of separating out the solid precipitate.
• the material separated from the sulphuric acid in the separator 20, comprises sludge containing from l% up to 10% mercury, and can be advantageously transferred to the roaster and there incinerated , since the invention enables a high mercury load to be placed on the acid system , and even presumes an accumulation of mercury in the acid in subsequent drying stages, where the major part of the elementary mercury present is taken -up.
• the sulphuric acid is taken from the separator 20, and passed through line 21 to the ci rculation ci rcuit 22, connected to the absorption tower 4. A small part of the sulphuric acid can also be removed in dilute form as product acid through line 47. If the amount of sulphuric acid passed through the circuit 22 B not sufficient to absorb the sulphur trioxide formed in the contact 3, water can be suppl ied to the circulation circuit through line 23.
• the sulphuric acid is passed through line 24, to the absorption tower 4, and from there to a circulation tank 26, and a circulation pump
• the sulphuric acid is passed from the after-drying tower 2 through line 35 to the ci rculation tank 36 and the circulation pump 37. If necessary, the acid is cooled indirectly in a cooler 38, water being I0 supplied to said cooler through line 39 and removed through line
• the sulphuric-acid concentration is held at 95-103% by introducing SO, directly in separate towers, which are in circulatory connection with t he washing tower, or directly to the second washing stage , through line 45 , or by supplying oleum from a separate oleum-production tower (not shown).
• the acid is cooled by means of a coolant in a cooling element 51 , in order to maintain a suitably low temperature.
• the acid can be diluted by supplying a part-flow of the moist sulphur-dioxide containing gas from line 5 to
• a separating means 53 which may have the form of a simple sedimentation vessel , a filter, or some other conventional separating means, where the mercury sulphate and/or mercury is separated out and removed, as indicated by the arrow 54.
• the thus diluted acid, which now contains less mercury, is removed, as illustrated by the arrow 55, and passed through the pre-drying tower I, through line 6, the remainder of the mercury present in the acid being precipated out in the acid- purifying stages 16 and 20, whereupon a mercury precipitate can be removed from the system or recycled in the process, for example , back to the roaster.
• a mercury precipitate can be removed from the system or recycled in the process, for example , back to the roaster.
• the high-mercury content of the circulation circuit 33 can also be maintained by supplying mercury-containing sludge to said circuit.
• Figure 3 is a diagram illustrating how a 98%-sulphuric acid having an original mercury content of about 2,000 g/t behaves when di luting to extremely low acid concentration at a temperature of about 20 C. Progressively more of the mercury content is precipitated as a product rich in mercury-sulphate by progressive dilution to approximately 65%. If the sulphuric acid is di luted still further, however, the precipitate wi ll re-dissolve and when dilution reaches 20% substantially the whole of the mercury content is redissolved.
• the mercury content throughout the diagram corresponds to grams per ton of original undiluted acid.
• the diagram also illustrates by means of a broken line how the solubility increases at sulphuric-acid concentrations in excess of about 80%.
• Figure 4 illustrates the solubility of mercury in 62%-acid as a function of temperature , and thus also illustrates how the solubility curves of Figure 3 are dependent upon the acid temperature. Thus, a temperature higher than 20 C displaces the curves slightly upwards
• Example I A similar gas to that described in Example I was charged, in similar amounts, to a plant of the kind according to Example I , but lacking a separate sulphuric-acid purifying ci rcuit. 16 tons of acid having a mercury content of 0.64 grams were passed each hour from the 80%-acid circuit directly to the absorption circuit 22. The same amountof acid as that recited in Example I (0.2 t/h) was bled-off from the 98% -acid circuit , and di luted in the same manner as in Example I , i.e. to an approximately 65%-acid and a mercury content of I00 g/t.
• Example 2a The same method as that recited in Example 2a was carried out, with the exception that the acid was diluted to 37%, which is the lowest content which can be tolerated in said system at prevai ling operating conditions. As a result of this further dilution with the addition of sulphur dioxide , it was possible to reduce the mercury content of the di luted acid to about I g/t. It was possible to produce in the plant the same amount of acid as that in example 2a, but with a mercury content of only 0.4 g/t.
• Example I illustrates that a highly pure sulphuric acid can be produced from a gas having mercury contents which are normal for pyrite roasting processes and that only about 35% of the mercury charged to the system need be precipitated with precipitating agents in the sulphuric acid purifying stage, whilst 60% is precipitated during the dilution stage.
• Example 2 illustrates that in the case of a gas having a mercury content according to Example
• Example 2a illustrates the result obtained with relatively moderate dilution
• Example 2b illustrates the maximum dilution which can be achieved without disturbing the liquid balance of the process.
• Example 3 illustrates the flexibility of the method, by applying said method to a roaster gas that is extremely rich in mercury and which is of the kind generated when roasting zinc concentrates rich in mercury. As will be seen, a highly pure product acid is obtained, despite the high mercury input.

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• 1982
  • 1982-12-03 SE SE8206917A patent/SE439153B/sv not_active IP Right Cessation
• 1983
  • 1983-12-01 EP EP19840900125 patent/EP0126763A1/de not_active Withdrawn
  • 1983-12-01 JP JP84500134A patent/JPS59502144A/ja active Pending
  • 1983-12-01 AU AU23378/84A patent/AU2337884A/en not_active Abandoned
  • 1983-12-01 WO PCT/SE1983/000420 patent/WO1984002123A1/en not_active Application Discontinuation

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