ZA200503124B

ZA200503124B - Conversion of a sulphur-containing waste material into a sulphur-containing product

Info

Publication number: ZA200503124B
Application number: ZA2005/03124A
Authority: ZA
Inventors: Johannes Philippus Maree; Deederick Johannes Theron
Original assignee: Csir
Priority date: 2004-04-30
Filing date: 2005-04-18
Publication date: 2006-01-25
Also published as: AU2005201759B2; AU2005201759A1

Description

EE e200, . THIS INVENTION relates to a process for the conversion of a sulphur- containing waste material into a sulphur-containing product. More particularly, the invention relates to a said process suitable for the treatment of a sulphur dioxide- containing gas, or gypsum, in particular, to produce a sulphur-containing product such as sulphur or sulphuric acid; and the invention relates also to an installation for carrying out the process.

According to one aspect of the invention there is provided a process for the conversion of a sulphur-containing waste material into a sulphur-containing product, the process comprising the steps of: reducing a sulphate compound to form a solid sulphide compound, converting the solid sulphide compound to hydrogen sulphide gas; and converting the hydrogen sulphide gas to a sulphur-containing product, the process producing a carbonate compound during the conversion of the solid sulphide compound to the hydrogen sulphide gas, and the process including the step, if necessary, of converting the waste material to the sulphate compound.

As indicated above, it is expected that the product will usually be sulphur although, naturally, it may instead be sulphuric acid. Furthermore, the carbonate compound produced during the conversion of the solid sulphide compound to the hydrogen sulphide gas is expected (after further processing) to be

Co oo Sa2005/05 104 of high purity, and in certain circumstances, as indicated hereunder, can form a valuable by-product. Indeed, depending on the primary reason for putting the process of the present invention into practice, the sulphur-containing product may ) sometimes be regarded as a by-product of a process whose primary purpose is to . treat and eliminate the sulphur-containing waste material.

The reducing of the sulphate compound to form the solid sulphide compound may be by thermal reduction, by heating the sulphate compound to a temperature in the range 800 — 1200°C in the presence of a reductant which reacts with the sulphate compound. In cases, mentioned hereunder, where the sulphate compound forms part of a mixture with a sulphite compound, the same thermal reduction step may be employed, the reductant reacting also with the sulphite compound. As will emerge hereunder, the sulphate compound will typically be an alkaline earth metal sulphate, such as calcium sulphate in particular, and it is to be noted that a suitable thermal reduction temperature for dealing with calcium sulphate, optionally admixed with calcium sulphite, is 1050°C.

Thus, the sulphate compound may be an alkaline earth metal sulphate, the reductant being a carbonaceous reductant containing carbon which reacts with the sulphate compound. This is typically in accordance with the reaction (1):

MSO, + 2C — MS + 2C0O (1)

When the sulphate compound is admixed with a sulphite compound, the sulphite compound will typically be an alkaline earth metal sulphite, which reacts with the carbon contained in the carbonaceous reductant in accordance with reaction (2):

MSO; + 1.5C — MS + 1.5CO, (2)

4 P2005 4310

In reactions (1) and (2) set forth above, M designates an alkaline earth metal, typically calcium, and when M is calcium and the MSO; is bound to water and is in the form of gypsum, reaction (1) can be expressed as reaction (3):

CaSO, 2 H,0 (gypsum) + 2C—CaS + 2CO, + 2 H;0 (3) . While this thermal reduction can be achieved by more or less conventional treatment in a kiln such as a rotary kiln, a shaft kiln or a fluoridized bed, it can also be carried out using a microwave oven.

Conveniently the thermal reduction takes place in a heated reactor such as kiln, in the presence of coal as the carbonaceous reductant, for example powdered coal which provides the carbon reductant in reactions (1), (2) and (3), solid sulphide and gaseous carbon dioxide and water vapour issuing from the heated reactor. Instead of coal, the carbonaceous reductant may be provided by activated carbon or oil, and indeed hydrogen may be used as a non-carbonaceous reductant.

The carbonaceous reductant used in the thermal reduction may be admixed with the sulphate compound in solid form, for example as a constituent of a dried sludge, upstream of the reduction step, to avoid the need for later solid/solid mixing thereof.

The converting of the solid sulphide compound to the hydrogen sulphite gas may be by reacting the solid sulphide compound with carbon dioxide and water in an aqueous solution of the solid sulphide compound, through which solution the carbon dioxide is bubbled as a gas, the carbon dioxide and water reacting with the solid sulphide compound to form the hydrogen sulphide gas and the carbonate compound, the hydrogen sulphide gas being stripped from the solution by the carbon dioxide gas. When the solid sulphide compound is an alkaline earth

A TE

- 5 Co metal sulphide and when the solution has a pH value greater than 8.3, the carbon dioxide may be bubbled upwardly through the solution to react in the solution with the alkaline earth metal sulphide according to reaction (4):

MS + CO, + HO — MCO3 + HS (4) . A solution comprising alkaline earth metal carbonate and bicarbonate issues from the alkaline earth metal sulphide conversion step, together with a gas stream comprising hydrogen sulphide and carbon dioxide, the carbon dioxide, while present in the alkaline earth metal sulphide conversion step, acting to lower the pH of the sulphide solution.

The major proportion of the hydrogen sulphide gas will be stripped out of the solution at pH values greater than 8.3 as summarised by reaction (4). The residual hydrogen sulphide gas will be stripped at pH values of 8.3 or less (as low as a pH of 6 or lower). In this low pH range the MS species will become more soluble and will appear in solution in the form of M(SH)(HCO;). The formation of this species is shown by reaction (4.2) and its conversion to HzS is shown by reaction (4.b):

MS + CO; + HO — M(SH)(HCO3) (4.2)

M(SH)(HCO3) + CO; + HO — M(HCO3)2 +H2S (4.b)

The carbon dioxide required can be obtained, for example, from the reduction step.

The solids content of the solution in the stripping step can vary relatively broadly, being, for example, 1-30% by mass. The reaction of the carbon dioxide with the alkaline earth metal sulphide may be promoted by using a venturi scrubber or a reverse-jet scrubber, to contact them with each other, or they may be passed through a packed bed reactor. The reaction of the alkaline earth metal sulphide to produce hydrogen sulphide, and the drogen sulphide stripping, both conveniently take place in the same reactor.

The converting of the hydrogen sulphide gas to a sulphur-containing - product may be to a product which comprises elemental sulphur, being by contacting the hydrogen sulphide gas with an aqueous iron (III) solution at a pH of about 3 to produce iron (II) and the elemental sulphur, the iron (II) being dissolved in the solution. (Various options exist whereby iron (III) can be recovered from iron (II) as mentioned hereunder). Instead, converting the hydrogen sulphide to sulphur may be by way of the so-called Clauss process. The Pipco process is another version of the

Clauss process which can be used for the production of sulphur from H>S — rich gas.

Another possibility is that the hydrogen sulphide may simply be burnt to form water and sulphur dioxide, which sulphur dioxide can then be converted via sulphurous acid (H.S 03) to sulphuric acid as a product.

Iron (III) can be converted to iron (II) via one or more of the following routes: electrolytic iron (II) - oxidation, in which case hydrogen gas is produced as a by-product; biological iron (II) - oxidation; or iron (II)- oxidation with oxygen at high pressure.

Furthermore, off-gas from the thermal reduction step can be used for sludge drying.

i 7

The sulphur-containing waste material may be sulphur dioxide in a sulphur dioxide-containing gas, the process including the step of converting the sulphur dioxide in the gas to a mixture comprising the sulphate compound and a sulphite compound. The converting of the sulphur dioxide to the mixture comprising the - sulphate compound and the sulphide compound may be by reacting the sulphur dioxide with a bicarbonate compound. The reacting of the sulphur dioxide with the bicarbonate compound may take place in an aqueous solution or slurry which also contains a carbonate compound in solution or slurry form; and the carbonate compound produced during the conversion of the solid sulphide compound to the hydrogen sulphide gas may, at least in part, be fed to the aqueous solution or slurry in which the sulphur dioxide reacts with the bicarbonate compound.

The bicarbonate compound may, for example, be an alkaline earth metal bicarbonate and the carbonate compound may accordingly be the corresponding alkaline earth metal carbonate. The sulphur dioxide may be reacted with a mixture of said bicarbonate compounds and carbonate compounds.

Preferably the bicarbonate compound and carbonate compounds are selected from the group consisting of strontium bicarbonate, strontium carbonate, barium bicarbonate, barium carbonate, magnesium bicarbonate, magnesium carbonate, dolomite (CaMg(COs).), calcium bicarbonate and calcium carbonate, and mixtures of any two or more thereof. Calcium bicarbonate and calcium carbonate are particularly preferred, being conveniently obtained from the carbonate compound produced when calcium sulphide as the solid sulphide compound is converted to hydrogen sulphide gas, for example, by bubbling carbon dioxide through water containing, dissolved therein, the solid sulphide compound, as described above.

The conversion of the sulphur dioxide may be by absorbing the sulphur dioxide in said aqueous bicarbonate compound-containing solution and/or slurry, and allowing it to react with the bicarbonate compound in the presence of oxygen according to - reactions (5) and (6):

SO; + M(HCO3); — MSOs + HO + 2CO; (5); and

SO, + %0, +M(HCO3); — MSO, + H20 + 2CO; (6)

In which M is an alkaline earth metal, preferably calcium. In this regard it should be noted that, in the case of an alkaline earth metal carbonate slurry, bicarbonate irons are produced in aqueous solution by contact between the carbonate and acid gas, such as sulphur dioxide, or by contact with an acid, such as sulphurous acid (H2S03), in solution.

The conversion may take place by passing the sulphur dioxide upwardly through the aqueous bicarbonate compound-containing solution or slurry, which also contains the carbonate compound in solution or slurry form, the sulphur dioxide-containing gas for example being a mixture of sulphur dioxide and air.

Carbon dioxide issues as a gas from the aqueous solution and/or slurry, sulphite and sulphate compounds being recovered as a solid mixture from the solution and/or slurry, for example by means of an optional settling step which may take place in a clarifier, and/or by an optional filtration step which take place in a filter press, any calcium sulphate typically being in the form of gypsum (CaS04.2H20).

The mixture so obtained may be subjected to an optional sludge-drying step prior to the reducing of the sulphate compound to form the solid sulphide a compound. In this regard it should be noted that the sulphite compound in the mixture will be reduced in the reducing step in substantially the same fashion as the reduction of the sulphate compound, and will also give rise to a solid sulphide compound. A portion of the filtrate from the filtration step may be discarded to waste . and replaced with fresh water to control the salt content in the clarifier by keeping the sodium iron content and the chloride iron content thereof at desirably low levels, to avoid any undesired build-up thereof.

The sulphur dioxide gas may be absorbed in the aqueous bicarbonate- containing solution or slurry, and may be converted to the sulphite compound/sulphate compound mixture in a single step, or in a plurality of like steps arranged in series, in which the solution and/or slurry on the one hand, and the sulphur dioxide-containing gas on the other hand, flow counter-current to each other to promote maximum absorption of sulphur dioxide gas in the solution and/or slurry, thereby to resist unwanted carry-over of carbonate to the thermal reduction step.

Carbonate in the thermal reduction step is undesirable as it decomposes to the corresponding oxide and carbon dioxide, calcium carbonate, for example, decomposes to lime and carbon dioxide. This oxide can find its way to the sulphur dioxide conversion step where it does not react with the sulphur dioxide but can, undesirably, react with the carbon dioxide formed. In the sulphur dioxide absorption and conversion, the gas and aqueous solution and/or slurry may be contacted together using a venturi scrubber or a reverse-jet scrubber, or they may be passed together through a packed bed reactor.

It should be noted that, while the aforegoing has been described primarily in relation to calcium as the alkaline earth metal in question, analogous reactions can be carried out for situations where the less preferred other alkaline earth metals magnesium, barium and indeed strontium are present instead of : calcium.

It should be noted that instead of mixing the carbonaceous reductant used in the thermal reduction upstream of the reduction step, e.g. with the dry sludge obtained by drying the sulphite/sulphate mixture from the sulphur dioxide conversion step, the carbonaceous reductant can be admixed with the carbonate compound produced in the step of converting the solid sulphide to the hydrogen sulphide gas, from which conversion step the carbonaceous reductant can be passed successively to the sulphur dioxide conversion step, and then to the thermal reduction step. Any losses of sulphate compounds such as gypsum or loss alkaline earth metal carbonate compounds such as limestone arising in the process can be made up by addition thereof to the process from external sources to the reduction step. The make-up of gypsum losses is particularly important when the emphasis of the present invention is shifted from the reduction of the sulphur dioxide content as a waste material in a gas to the production of sulphur, in which case such sulphur dioxide from such gas can be at least partially supplemented by gypsum, e.g. from existing gypsum waste dumps.

It should also be noted that carbon dioxide from the conversion step whereby the sulphur dioxide is converted to sulphate and sulphite can be used in reactions (4), (4.a) and (4.b); and the carbonate produced in reaction (4) is available to provide the carbonate and bicarbonate for reaction with sulphur dioxide in accordance with reactions (5) and (6).

Instead of sulphur dioxide, the waste material may comprise the : sulphate compound which is reduced to form the solid sulphide compound. This version of the process may also include the step of recovering, as a by-product, the carbonate compound produced during the conversion of the solid sulphide compound to the hydrogen sulphide gas, as the carbonate compound will be of high purity. This version of the process is particularly suitable for producing valuabie sulphur-containing products from gypsum waste dumps which can be an environmental embarrassment.

The invention extends also to an installation for carrying out the process defined and described above, the installation comprising: a solid sulphide compound production stage for reducing a solid sulphate compound to a solid sulphide compound; a hydrogen sulphide production stage for converting said solid sulphide compound to hydrogen sulphide gas; and a product production stage for converting the hydrogen sulphide gas to a sulphur-containing product, the solid sulphide compound production stage feeding via a solid sulphide compound feed line into the hydrogen sulphide production stage, and the hydrogen sulphide production stage feeding via a hydrogen sulphide gas feed line into the product production stage.

The installation may nolude s sulphur dioxide conversion stage for converting a sulphur dioxide gas feed to a mixture of sulphate and sulphite compounds, the sulphur dioxide conversion stage feeding via a sulphate compound feed line into the solid sulphide compound production stage.

The arrangement of the installation emerges in more detail from the description of the process of the invention set forth above, and from the following detailed description in which the invention is described by way of illustrative, non- limiting example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows a schematic flow diagram of an installation for carrying out the process of the present invention; and

Figure 2 shows a similar schematic flow diagram of another installation for carrying out the process of the present invention.

Referring first to Figure 1 of the drawings, reference numeral 10 generally designates an installation for carrying out the process of the present invention. The installation 10 comprises a sulphur dioxide conversion stage in the form of a packed scrubbing tower 12 fed by a gas feed line 14 and by a flow line 16, the tower 12 having a gas outlet line 18 and an overflow outlet flow line 20, leading to a settling stage provided by a clarifier 22. The clarifier 22 is provided with an outlet overflow line 24 and a sludge outlet line 26 leading to a filtration and drying stage provided by a filter press/sludge dryer combination 28, from which a dried sludge flow line 30 issues. Flow lines 20, 26 and 30 together form a sulphate compound food line feeding from the sulphur dioxide conversion stage 12 to the solid sulphide compound production stage 32.

Flow line 30 leads to a thermal reduction stage in the form of a kiln 32 provided with a coal feed line 34. The thermal reduction stage 32 acts as a solid sulphide compound production stage for producing solid sulphides.

The kiln 32 feeds via a solid sulphide compound feed line 36 and a carbon dioxide feed line 38, to a hydrogen sulphide production stage in the form of a packed tower 40. Line 36 feeds into tower 40 at a high level, while line 38 feeds into tower 40 at a low level. Flow line 16 issues at a low level from tower 40, and a gas flow line 42 issues at a high level from tower 40, a further gas flow line 44 entering the bottom of the tower 40. A water feed line 45 is shown feeding into the top of the tower 40.

A product production stage is designated 46, flow line 42 entering the top of the stage 46 and flow line 44 issuing from the top of stage 46, the flow line 42 thus forming a hydrogen sulphide gas feed line to the product production stage 46.

Stage 46 has a sulphur product outlet flow line 48 and is connected to an oxidation stage 50 by flow lines 52 and 54 respectively, flow line 52 issuing from stage 50 and feeding into stage 46, and flow line 54 issuing from stage 46 and feeding into stage 50. An electric power supply line 52 is shown for providing power to oxidation stage

In Figure 2, the same sferonre numerals designate the same parts as in Figure 1, unless otherwise specified. Differences between Figure 1 and Figure 2 are that Figure 2 omits gas flow line 44 but includes an air feed line 58 feeding into stage 46 which is a combustion stage, flow line 52 being omitted; and flow line 48 : issues from stage 50 rather than from stage 46. Flow line 60 passes from flow line 42 into stage 50, stage 50 being a product production stage for producing sulphur according to the Clauss process, rather than being an oxidation stage as is the case in Figure 1.

Reverting to Figure 1, and in accordance with the process of the present invention, an air/sulphur dioxide gas mixture is fed along feed line 14 into the bottom of tower 12, the top of which tower 12 is fed along flow line 16 with a calcium carbonate/calcium bicarbonate aqueous solution/sludge mixture. In tower 12 reactions (1) and (2) take place, with the production of calcium sulphate, calcium sulphite, water and carbon dioxide. Carbon dioxide issues from tower 12 along flow line 18, whereas an aqueous solution/slurry issues from tower 12 along flow line 20, containing calcium sulphate and calcium sulphite. In the clarifier 22 precipitation of calcium sulphite takes place, together with precipitation of gypsum (calcium sulphate dihydrate) at temperatures less than 40°C. At temperatures above 40°C precipitation of calcium sulphate hemihydrate (CaSO, 2H,0) takes place. Clarified aqueous overflow issues from clarifier 22 along flow line 24 to waste, and calcium sulphite/gypsum sludge issues from clarifier 22 along flow line 26 to the filter press/sludge dryer combination 28.

The sludge is filtered and ed in said combination 28, dried sludge issuing therefrom along flow line 30 to kiln 32. Powdered coal is added to kiln 32 along flow line 34, and in kiln 32 thermal reduction of the calcium sulphate and calcium sulphite in the sludge takes place, leading to the production of calcium sulphide and carbon dioxide in accordance with reactions (1) - (3). The calcium sulphide passes along line 36 to the top of tower 40, the carbon dioxide passing flow line 38 to the bottom of tower 40. In tower 40 reaction (4) takes place, the calcium sulphide reacting with carbon dioxide and water fed along line 45 to produce calcium carbonate (solid) and hydrogen sulphide (gas), and also some calcium bicarbonate (dissolved). Gas comprising carbon dioxide and hydrogen sulphide stripped thereby from the aqueous contents of the tower 40 issues from tower 40 along line 42 to stage 46, while an aqueous calcium carbonate/calcium bicarbonate solution/slurry issues from tower 40 along flow line 16 to scrubbing tower 12. This slurry contains the calcium bicarbonate as a potentially valuable by-product, after removal of impurities mixed therein and arising from the coal and from the gypsum.

In stage 46 hydrogen sulphide reacts with ferric ions from flow line 52 in accordance with reaction (7):

H,S + 2Fe®* — S+2Fe** + 2H" (7)

Sulphur issues from stage 46 along flow line 48 and ferrous ions from reaction (7) issue from stage 46 along flow line 54 to oxidation stage 50. Carbon dioxide entering stage 46 from line 42 returns to tower 40 along line 44. In oxidation stage 50 the ferrous ions are electrically oxidised in accordance with reactions (8) and (9): 2Fe?* — 2Fe® + 2e (8) 2H,0 + 2e" — Hy + 20H" (9)

The electric power for the electrolytic oxidation is fed to stage 50 along power line 56. Instead, the so-called Pipco process can be used to obtain sulphur from the

H.S. . Any solid calcium carbonate issuing from tower 12 with the solution/slurry along flow line 20 settles in the clarifier 22, and passes together with the calcium sulphite/gypsum sludge to the kiln 32, where it is burnt to CaO, the CaO then reacting with CO; in tower 40 to form CaCO:;.

Operation of the process with reference to Figure 2 is substantially similar to that described above with reference to Figure 1, except that the Clauss process for the production of sulphur replaces the electrolytic process described with reference to Figure 1.

With reference to Figure 2, substantially a third of the flow along flow line 42 enters stage 46, together with air along line 58, the remaining flow from flow line 42 passing along flow line 60 to stage 50.

In stage 46 hydrogen sulphide reacts with oxygen in accordance with reaction (10):

HoS + 1.50, — SO2 +H 20 (10)

The sulphur dioxide produced issues from stage 46 along flow line 54 to stage 50. In stage 50 hydrogen sulphide reacts with the sulphur dioxide from flow line 54 in accordance with reaction (11): 2H,S + SO, —» 3S +2 H ,0 (11)

thereby producing sulphur as a product, nich issues from stage 50 along flow line

In refinements of the process described above any losses of the iron . required for reactions (7) and (8) can be made up by the addition of ferrous sulphate and/or ferric sulphate. While hydrogen sulphide will not usually be present in the gas feed along line 14 into tower 12, the process is indeed capable of eliminating any hydrogen sulphide present in the gas feed along line 14, the hydrogen sulphide being oxidised in tower 12 in the presence of oxygen to sulphuric acid, which in turn reacts with calcium bicarbonate to form calcium sulphate dihydrate or hemihydrate precipitate, depending on the temperature. When dolomite is used instead of calcium carbonate for the slurry, the presence of magnesium cations in the slurry means that magnesium bicarbonate can be recovered from solution, after stripping of carbon dioxide from the magnesium carbonate. This magnesium bicarbonate can be used for the production of magnesium metal as a valuable by-product. Gypsum losses and calcium carbonate losses will require to be supplemented from time-to- time, as needed, from external sources, typically by feeding thereof into flow line 16 and the process can thus provide a facility for consuming gypsum from gypsum dumps, for example produced by fertilizer manufacturing, to obtain calcium carbonate and sulphur; and off-gas from the kiln 32 can in principle be used for sludge-drying in the filter press/sludge dryer combination 28. In this case the gypsum dumps are regarded as comprising a sulphur-containing waste material and elimination thereof can be the primary purpose of operating the process of the present invention, instead of eliminating sulphur dioxide as a sulphur-containing waste material from the gas feed in line 14. In this case it is in principle feasible to eliminate the tower 12, clarifier 22 and iter press/sludge dryer combination 28, line 16 thus feeding directly into line 30.

It is an advantage of the present invention that sulphur dioxide gas from smelters, power stations or the like, which is a major source of pollution and can cause acid rain damaging to forests on a global scale, is consumed and is converted to sulphur as a valuable by-product, with reference to the examples described for the installations of Figures 1 and 2. The process can consume surplus gypsum from power stations, and uses limestone and water which are freely available. The sulphur can be melted and refined if necessary, and it is an important feature of the invention that gypsum is converted into calcium sulphide which is in turn converted to the calcium carbonate raw material and sulphur by-product.

Claims

1. A process for the conversion of a sulphur-containing waste material into a sulphur-containing product, the process being characterized in that it comprises the steps of: reducing a sulphate compound to form a solid sulphide compound, converting the solid sulphide compound to hydrogen sulphide gas; and converting the hydrogen sulphide gas to a sulphur-containing product, the process producing a carbonate compound during the conversion of the solid sulphide compound to the hydrogen sulphide gas, and the process including the step, if necessary, of converting the waste material to the sulphate compound.

2. A process as claimed in Claim 1, characterized in that the reducing of the sulphate compound to form the solid sulphide compound is by thermal reduction, by heating the sulphate compound to a temperature in the range 800 — 1200°C in the presence of a reductant which reacts with the sulphate compound.

3. A process as claimed in Claim 2, characterized in that the sulphate compound is an alkaline earth metal sulphate, the reductant being a carbonaceous reductant containing carbon which reacts with the sulphate compound.

4. A process as claimed in any one of the preceding claims, characterized in that the converting of the solid sulphide compound to the hydrogen sulphide gas is by reacting the solid sulphide compound with carbon dioxide and water in an aqueous solution of the solid sulphide compound rough which solution the carbon dioxide is bubbled as a gas, the carbon dioxide and water reacting with the solid sulphide compound to form the hydrogen sulphide gas and the carbonate compound, and the hydrogen sulphide gas being stripped from the solution by the carbon dioxide gas.

5. A process as claimed in any one of the preceding claims, characterized in that the converting of the hydrogen sulphide gas to a sulphur-containing product is to a product which comprises elemental sulphur, being by contacting the hydrogen sulphide gas with an aqueous iron (III) solution at a pH of at most 3 to produce iron (II) and the elemental sulphur, the iron (11) being dissolved in the solution.

6. A process as claimed in any one of the preceding claims, characterized in that the sulphur-containing waste material is sulphur dioxide in a sulphur dioxide- containing gas, the process including the step of converting the sulphur dioxide in the gas to a mixture comprising the sulphate compound and a sulphite compound.

7. A process as claimed in Claim 6, characterized in that the converting of the sulphur dioxide to the mixture comprising the sulphate compound and the sulphite compound is by reacting the sulphur dioxide with a bicarbonate compound.

8. A process as claimed in Claim 7, characterized in that the reacting of the sulphur dioxide with the bicarbonate compound takes place in an aqueous solution or slurry which also contains a carbonate compound in solution or slurry form.

9. A process as claimed in Claim 8, characterized in that the carbonate compound produced during the conversion of the solid sulphide compound to the hydrogen sulphide gas is, at least in part, fed into the aqueous solution or slurry in which the sulphur dioxide reacts with the bicarbonate compound.

10. A process as claimed in any one of Claims 1 — 5 inclusive, characterized in that the waste material comprises the sulphate compound which is reduced to form the solid sulphide compound.

11. A process as claimed in Claim 10, characterized in that it includes the step of recovering, as a by-product, the carbonate compound produced during the conversion of the solid sulphide compound to the hydrogen sulphide gas.

12. An installation (10) for carrying out the process as claimed in Claim 1, the installation being characterized in that it comprises: a solid sulphide compound production stage (32) for reducing a solid sulphate compound to a solid sulphide compound; a hydrogen sulphide production stage (40) for converting said solid sulphide compound to hydrogen sulphide gas; and a product production stage (46, 50) for converting the hydrogen sulphide gas to a sulphur-containing product, the solid sulphide compound production stage (32) feeding via a solid sulphide compound feed line (36) into the hydrogen sulphide production stage (40), and the hydrogen sulphide production stage (40) feeding via a hydrogen sulphide gas feed line (42, 60) into the product production stage (46, 50).

] «2005/7031 24

13. An installation (10) as claimed in Claim 12, characterized in that is includes a sulphur dioxide conversion stage (12) for converting a sulphur dioxide gas feed to a mixture of sulphate and sulphite compounds, the sulphur dioxide conversion stage - (12) feeding via a sulphate compound feed line (20, 26, 30) into the solid sulphide compound production stage (32). DATED THIS 18" day of APRIL 2005 / AV vR SCHWEIZER N ADAMS & ADAMS APPLICANT'S PATENT ATTORNEYS