EP0127664A1 - A method for producing pure sulphuric acid and a high-grade mercury product from gases that contain sulphur dioxide and mercury - Google Patents

Abstract

The present invention describes a process for the manufacture of sulfuric acid from practically dry gas containing sulfur dioxide and produced by the treatment of intermediate products and raw materials containing mercury, in order to obtain pure sulfuric acid. and a product with a high mercury content. The gas is purified from the accompanying gaseous mercury by bringing the gas into contact with sulfuric acid in one or more stages (1, 2), so that most of the mercury is absorbed and dissolved in l concentrated sulfuric acid having a concentration of at least about 90%; the purified gas is then supplied to a contact installation (4) for the production of pure sulfuric acid. The characteristic of the invention is to circulate the concentrated sulfuric acid in a circuit which comprises at least one drying tower (2) for the gas containing sulfurous oxide, a sludge separator (19) and a reactor d 'oxidation (3). The elemental mercury gas accompanying the gas in the drying tower (3) is oxidized and absorbed in the acid, and isolated mercury compounds are separated from the acid and removed from the circuit. The acid purified in this way is then brought into contact with an oxidant in the oxidation reactor (3), so that the dissolved mercury present in the acid is oxidized to mercury ions (11) which are then used in the drying tower (2) to initiate the oxidation of elemental mercury.

Description

A METHOD FOR PRODUCING PURE SULPHURIC ACID AND A HIGH-GRADE MERCURY PRODUCT FROM GASES THAT CONTAIN SULPHUR DIOXIDE AND MERCURY

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%.

By 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.

More and more attention is now being paid to the presence of mercury in industrial processes, and the risks associated therewith. As wi l l be understood, the problems involved become more significant when the products concerned are fertilisers and foodstuffs. Extremely large quantities of sulphuric acid are used in the manufacture of these products, and consequently, the sulphuric acid should contain as little mercury as possible. Mercury is also liable to be introduced into process or product chains through other processes which have no direct connection with said product, giving rise to obvious risks of contamination.

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, pyπ^'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.

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. Despite this, however, it is impossible to control the roasting and gas-cleansing processes in a manner to ensure that the amount of elementary gaseous mercury contained in the dust- freed gas is sufficiently low to enable the gas to be used in other processes, or to be discharged to a recipient. Consequently, mercury vapour accompanies the gas through the whole of the sulphuric-acid process, and is finally absorbed in the end product, i.e. the sulphuric acid. This contaminates the acid and also represents a loss of valuable mercury.

In an attempt to alleviate these problems, various purifying processes have been proposed, in accordance with which , either the sulphuric acid is treated in a manner to precipitate its mercury content, or the roaster gas is washed, or treated in some other way, to eliminate its mercury content, prior to passing the gas through the sulphuric- acid plant. The choice between these two alternatives, i.e. sulphuric-acid purification or roaster-gas washing is primarily decided by local conditions, for example, by whether or not the existing relevant plants are available, whether or not the equipment available is good enough to effect a wet-gas cleansing process satisfactorily, or whether or not the space avai lable is sufficient to carry out the cleansing process. The alternative selected is also determined by the level of the mercury present and the variations in said level. In many cases it has been found that those methods which include the purification of the sulphuric acid are to be preferred.

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. When carrying out such purifying processes, it is not permitted for other toxic substances, such as lead and arsenic, to be introduced into the acid. In certain cases, 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.

The 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, however, 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. Subsequent to separating the solid mercury-containing substance by filtration , 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. In addition , since large volumes of acid must be treated, 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.

Methods have also been proposed in which the tendency of the sulphuric acid to dissolve mercury, at least at high acid concentrations, is utilised for cleansing roaster gases. One such method , in which the gas is cleansed in a single stage at elevated temperature with a sulphuric acid concentration of 85-90% is described , for example in DE-8-2 132 231.

Another method, in which the gas can be cleansed in several stages, is described in DE-B-2 243 577. According to this method, mercury is first removed from moist roaster gas, by washing the gas with sulphuric-acid washing solutions in conjunction with drying the gas, and then precipitated from the sulphuric-acid solutions , in sulphide or selenide form , in certain cases in conjunction with the reduction of certain metals. The washing acids used have concentrations of between 30 and 99%. Mercury is not absorbed to any noticeable extent, unti l an acid concentration of about 70% H„SO _ is reached. 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 792573 in conjunction with a method for avoiding the manufacture of 'black ' acid from roaster gases that contain organic compounds.

A further development of the aforementioned processes which utilise sulphuric acid as a medium for eliminating mercury vapour from roaster gases is described in our earlier Patent Specification SE-B

7 307 048-4, in accordance with which concentrated sulphuric-acid which contains mercury is also supplied and utilised for mercury- absorption in the fi rst stage , and then purified. As previously indicated, since when carrying out the method all mercury supplied to the process, both through the gas and through contaminated sulphuric acid, shall be removed by treating acids from the first process circuit in a manner to precipitate the mercury contained in said acids, the previously mentioned separation problems wil l occur, unless the sulphuric-acid purifying stages are dimensioned with respect hereto. Because of this, the investment costs and operation costs are often prohibitive, at least when large quantities of mercury are to be charged to the system.

Thus, 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. As beforementioned, 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. In addition, 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. Furthermore, 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. At present, 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.

Surprisingly, it has now been found possible to provide a method by which the aforementioned problems encountered when using mercury- containing gases that also contain sulphur-dioxide for producing sulphuric acid are substantially eliminated. The method also enables the major part of the gaseous mercury in the gas 'to be recovered , in the form of a high-grade mercury product, which can be worked-up into useful mercury products, without requiring large investments to be made. The method can also be applied advantageously for working-up or refining mercury-containing intermediate products into high-grade products, for example, such intermediate products as those obtained when separating particulate mercury compounds from mercury-contaminated gases in gas-cleansing systems.

As mentioned in the introduction, 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.

Thus, when carrying out the method according to the invention 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. As a result, the acid will contain more mercury (II) ions. In turn , these ions contribute towards oxidizing elementary mercury in the gas to mercury ( II) ions, which in turn are dissolved in the acid in large quantities. Mercury salts, primari ly Hg-SO . , are precipitated when the circuit is saturated with respect to mercury. These mercury salts can be readi ly isolated in conventional sludge separators incorporated in the circuit. Thus, by oxidizing the acid, it will contain more mercury (II) ions than it otherwise would after having passed through the drying tower, and hence the effect of oxidation on ^elementary mercury accompanying the gas will be improved. In this way, larger input quantities of mercury can be permitted in the final- drying stage or stages having acid contents above about 90%, and preceding the contact stages in a sulphuric-acid plant which operates in accordance with the contact method. This means that in a multi¬ stage process, for example, of the double-dryer type, the mercury load on the initial stage or stages, in which the acid is more dilute , can be reduced and thereby contribute to reducing filtering require¬ ments.

The invention wi ll now be described in more detail with reference to the accompanying drawings and the following examples, in which drawings 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.

In the context intended. , by '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. Thus, at acid concentrations above 70% the solubility increases strongly with increasing acid concentrations and at 100% is about 30,000 g/t. In the main, 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%. When seen against this background it will be understood why oxidization of the concentrated acid can be effective in the removal of mercury from said acid. If the concentrated acid contains a level of both Hg⁺ and Hg ⁺ which lies close to the saturation limit, when the acid is oxidised, 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. Thus, by oxidizing elementary mercury, 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. Examples of such powerful oxidants include halogens, hydrogen peroxide , oxygen, ozone and ammonium peroxodisulphate. Preferably 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. In this respect, when using 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. In certain cases, it may be desirable to amplify the oxidation effect in the separate absorption tower, by also charging an oxidant to the drying tower, 'or by recycling to the ci rcuit a part -flow of separated sludge, subsequent to oxidizing the same.

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. Thus, 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. It is particularly advantageous, to apply the invention in a multi-stage gas-purifying and sulphuric-acid purifying-plant of the so-called double-dryer type, for example, such systems as those described in SE,B-7307048-4 and DE-A-2 243 577, whereupon these plants can be improved and made more effective. . This also increasesthe possibi lities of working-up materials rich in mercury, for example mercury-rich sludge obtained from gas- purifying systems, since the load on the acid-purifying section will substantially decrease. This means that less precipitating agent, for ^• example, thiosulphate, is consumed, and that filtering problems decrease, thereby enabling bottle-necks in these purifying systems to be avoided. In addition , filtering resources can be reduced, in relation to present levels. The mercury load on the acid-purifying stages normally varies in dependence upon variations in the amounts of mercury contained by the raw materials. Such problems can also be mastered effectively and the variations smoothed when practising the invention.

A preferred embodiment of the invention will now be described with reference to the process schematic shown in Figure 2.

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. When 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. In the drying circuit I , 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. As a result, the concentration of the acid falls, although efforts are made not to permit the concentration to fall beneath about 70-85%, since otherwise the water-vapour pressure above the acid would be too high to achieve effective drying. A sulphuric-acid concentration of about 80% is normally ^"sought in this circuit. About 10% of the

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. Subsequent to being stripped of sulphur dioxide , 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. In this ci rcuit , the acid is cleansed of mercury compounds

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precipitated in the drying circuit 2, by separation in the sludge separator I9A , whereafter part of the sludge-freed acid 8A is optionally passed to the stripper 12 through line 9, while the remainder of the acid is passed to the absorption circuit 3, through line 21 , in which circuit the sulphur dioxide accompanying the acid is stripped therefrom , by injecting sulphur trioxide into the lower part of the tower associated with said circuit 3, through a line 45, to which line sulphur trioxide can be passed from the converter 4, through the line 43, the stripped sulphur dioxide, together with surplus sulphur trioxide, being removed through a line 42A and admixed with the gas obtained from the after-drying circuit 2 in line 42, said line conducting the gas to the converter 4, for oxidizing sulphur dioxide. At the same time, 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.

These salts are isolated in the form of a high-grade sludge in the sludge separator 19. If so desired , it is possible, by intensive oxidation, to recover substantially all the recoverable amount of mercury in the separator 19, for example when the product containing the mercury-(l!)-precipitate can be used directly. In such cases, the drying process in ci rcuit 2 will not result in any substantial mercury precipitation , since the Mercury (l)-content will lie wel l beneath the saturation limit. Thus, the acid passed from the absorption ci rcuit 3 and the separator 19 through lines 18 and 20 , will have an elevated content of mercury (II) ions, in relation to the acid passed to the absorption circuit through lines 22 and 21.

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

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:__ more zones in an external, common tower. A strongly oxidizing environment rich . in mercury ( II) ions is maintained in the circuit 2, in order to facilitate the absorption and oxidation of mercury. The concentration of H„SO_ in this ci rcuit shall be held above 90%

- 4 and preferably at 95-99%, by supplying concentrated 98.5%-acid from the absorption circuit of the plant,and by the direct introduction of SO- into said separate absorption circuit 3, which is thus in ci rculatory connection with the drying circuit. When the ci rcuit incorporates two towers , 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. Alternatively , 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. This results in a certain degree of mercury oxidation while enabling, at the same time, sulphur dioxide accompanying the sludge to be drawn-off and introduced, together with the oxidizing ai r, through lines not shown , into gases from the ci rcuit 2. If desired , 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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Claims

uCLAIMS

1. In the manufacture of sulphuric acid from substantially dry gas which contains sulphur dioxide and which is obtained when treating mercury-containing raw materials and intermediate products, a method for obtaining pure sulphuric acid and a high-grade mercury product, in which method the gas is cleansed of accompanying gaseous mercury by contacting the gas with sulphuric acid in one or more stages, so that the major part of the mercury is taken-up and dissolved in concentrated sulphuric acid having a concentration of at least 90%, whereafter the purified gas is passed to a contact plant for producing pure sulphuric acid, characterised by circulating the concentrated sulphuric acid in a ci rcuit comprising at least one drying tower for the sulphur-dioxide-containing gas, a sludge separator, and an oxidation reactor; by oxidizing gaseous elementary mercury accompanying the gas in the drying tower and absorbing said mercury in said acid; by isolating separated mercury compounds and removing them from the ci rcuit; by bringing the thus purified acid into contact with an oxidant in the oxidation reactor, so that dissolved mercury present in the acid is oxidized to mercury (II) ions; and by utilising said mercury (II) ions in the drying tower to promote the oxidation of elementary mercury.

2. A method according to claim I , characterised by maintaining in the drying tower or towers, an acid concentration of 90-99%, preferably 98-98.5%.

3. A method according to claim 2, in which a plurality of drying towers are utilised in the circuit, characterised by maintaining in the first tower an acid concentration of 90-95% and maintaining in the last tower of said plurality of towers an acid concentration of 95-99%.

4. A method according to any one of claims 1-3, characterised in that oxidation in the oxidation reactor is effected with sulphur trioxide or oleum.

5. A method according to any one of the preceding claims, characterised in that at least a part of the mercury compounds removed from the ci rcuit is oxidized and then returned to the drying tower.