Background Liquid and dissolved forms of sulphur salt are released in many different processes. An example of a dissolved oxidised sulphur salt is sodium sulphate. This is commonly formed when sulphuric acid is neutralised with caustic soda. Under many circumstances, such highly concentrated salt streams cannot be released into the environment.

Consequently, at the expense of huge energy consumption, water is evaporated to obtain a solid salt. Another common way to treat such a stream is by adding a calcium salt, resulting in the formation of gypsum (CaS04. 2H20).

An example of material that contains reduced sulphur is the slag that is formed by certain smelting processes. Such slag contains among others sodium sulphide. This slag is unstable against leaching when land filled. Another example of a stream with reduced sulphur is spent caustic (hydrogen sulphide dissolved in sodium hydroxide-solution).

A process for eliminating sulphur dioxide or hydrogen sulphide from gases through absorption of these sulphur compounds in an alkaline washing liquid containing (bicarbonate, optionally reducing sulphite and sulphate to sulphide and biologically oxidising the sulphide salts to elemental sulphur is described in WO 92/10270. The biological oxidation is carried out after adjusting the concentration of buffer to 20-2000 meq/1 (in case of sodium carbonate : 1-70 g/l = 0. 02-1. 3 M Na) and at a pH of 6-9. The resulting desulphurised liquid containing carbonate and bicarbonate is returned to the gas washing step.

WO 97/43033 discloses the use of particular alkaliphilic bacteria in oxidising sulphide to elemental sulphur at a pH of 9-11, using lye or soda where necessary for pH adjustment.

Description of the invention It has been found that sulphur-containing salts, such as sodium sulphate and sodium sulphide waste and slags, can be treated efficiently to produce elemental sulphur and sodium carbonate and bicarbonate, each of which can be useful materials. Thus, the invention pertains to a process of desalination and desulphurisation and recovery of sodium (or other) salts and sulphur. The process converts alkali metal salts of oxidised and reduced sulphur (e. g. sodium sulphate, sodium sulphite, sodium sulphide etc.) to

elemental sulphur and dissolved carbonate, followed by the recovery of the carbonate as the solid alkali metal carbonate salt.

An important step in this process is the biological partial oxidation of the sulphide to elemental sulphur according to the following equation: Na + HS'+ 1/2 Oz--3 Na+ + S° + OH- (1) where sodium (Na+) can also be another alkali metal such as potassium or lithium, or alkaline earth metal, or a mixture of sodium with such other metal (s). The conversion can be carried out at a total cation concentration up to 8 M, preferably in the range of 2 to 4 M. In case of sodium, a particularly useful concentration is between 2 and 6.4 M.

Metal cation concentrations can be measured by standard methods, including e. g.

Inductive Coupled Plasma, after suitable dilution where necessary.

The conversion can be carried out in the range from pH 7 to pH 13, though the preferred pH is from pH 9 to pH 12, especially about 10. 5. The reaction is carried in a bioreactor, and, in principle, the reaction can occur between 0 and 100°C, though preferably at 25- 35°C. The pH can be adjusted using the redox potential of the medium as described in W098/04503 ; thus the redox potential can be adjusted between-300 and-350 mV using a platinum-coated electrode and an Ag/AgCl reference electrode.

The bacteria required for the biological oxidation of sulphide should be resistant to and active under the alkaline pH and the high salt concentrations. Suitable bacteria are halo- alkaliphilic sulphide-oxidising bacteria. Several of such bacteria are known and have been obtained from soda lakes. They include the genera Thifo) alkalivibrio, Thialkali- microbium, Thi (o) alkalispira and Thioalkalicoccus. Examples are Thialkalivibrio versutus (DSM 13738) (AL2), Tv. nitratis (DSM 13741), Tv. denitrificans (DSM 13742), Tv. paradoxus (DSM 13531), Tv. thiocyanoxidans (DSM 13532), Tv. jaranasclaii (DSM 14478), Thialkalimicrobium aerophilum (DSM 13739) (AL3), Tm. sibiricum (DSM 13740), Tm. cyclicum (DSM 14477) and Thialkalispira microaerophila (DSM 14786). Moderately haloalkaliphilic sulphur-oxidising bacteria and their use in oxidation of sulphide have also been described in W097/43033.

The alkalinity that is formed during this conversion can be neutralised by an acid, in particular carbon dioxide (CO2), in order to keep the pH in the reactor constant. When pH control with carbon dioxide is applied, the overall conversion can be written as follows (neglecting the formation of biomass) : Na+ + HS-+ 1/2 °2 + C02-Na+ + S° + HCO3- (2) Bicarbonate (HCOs') is in equilibrium with carbonate (CO32-) :

The actual concentration of both species depends on the pH.

The elemental sulphur (S°) is separated from the liquid as a solid. It may be disposed or used for the production of sulphuric acid. The liquid resulting after separation of elemental sulphur is loaded with a bicarbonate/carbonate solution, and is then led to a device that recovers the bicarbonate/carbonate as a solid, especially as sodium carbonate and/or bicarbonate. Several methods for recovering carbonate and bicarbonate salts are possible, including the following: 1) Water is evaporated and a mixture of sodium carbonate and sodium bicarbonate is recovered. This is the least preferred method of the four, as it requires a lot of energy.

2) The pH of the liquid is lowered, preferably with carbon dioxide (CO2). The pH drop and the presence of additional dissolved C02 results in an increase of the dissolved bicarbonate: 3 Na+ + HCO3-+ co32-+ CO2 (g) + H2O <==> 3 Na+ + 3 HCO3- As the solubility of sodium bicarbonate, certainly per sodium ion, is much lower than the solubility of sodium carbonate, it is possible in this way to supersaturate the solution with sodium bicarbonate, resulting in the crystallisation of this compound. This can be done by decreasing the pH to between 5.5 and 11, preferably to between 6 and 9, especially about pH 7. The solid sodium bicarbonate can be separated from the liquid and dried. If required, the sodium bicarbonate can be converted to sodium carbonate by heating the solid until water vapour and carbon dioxide are released. These compounds can be recycled back to the process, as a liquid and/or as a gas.

3) By lowering the temperature, a supersaturation of sodium carbonate can be obtained.

This is possible because the solubility of sodium carbonate has a much more pronounced temperature dependency than the solubility of sodium bicarbonate. For example, the solubility of sodium bicarbonate drops from about 1.1 to about 0. 8 M upon cooling from 30° to 10°C, while the solubility of sodium carbonate drops from about 1. 35 to about 0.45 M upon the same cooling. This method is especially favourable when the conversion of reaction (2) has occurred at the upper range of the indicated pH range (pH 6 to 13). The temperature can be lowered from the temperature at which reaction (2) has occurred down to close to the freezing point of water, e. g. to between 0 and 10 °C.

4) By raising the pH to between about 11 and 13, together with lowering the temperature to about 10°C (e. g. between 5 and 15°C). In this way, sodium carbonate is obtained above its solubility limit, and thus precipitates. This method has the advantage of saving a calcination step when producing solid sodium carbonate.

The methods above can also be combined for an optimum result, i. e. the temperature is lowered, some water is evaporated and the pH is lowered. The liquid remaining after the crystallisation of solid sodium carbonate or bicarbonate (supernatant) can be used for dissolution of material containing solid sodium sulphide or for dilution or pH adjustment of the influent sulphur containing stream. Although sodium is the preferred cation to be removed and recovered, the process can also be applied to other metals such as potassium ; in this case, higher concentrations and/or lower temperatures are applied in the salt recovery steps as described above, because of the higher solubility of potassium (bicarbonate ; e. g. a factor of 1.5 or 2 on. concentrations, or a temperature decrease of 10°C, or a pH adjustment.

Although in most applications it is preferred to separately recover elemental sulphur and sodium (bicarbonate, it is also possible to combine the precipitation of elemental sulphur and (bicarbonate, e. g. in small-size desulphurisation plants. This can be done by adding carbon dioxide before the separation of sulphur, e. g. immediately after the bioreactor, or, preferably while biologically oxidising the sulphide, i. e. in the bioreactor.

This allows the process to be carried out in a simplified manner, applying conditions in the bioreactor, especially a pH between about 7 and 9 e. g. resulting fiom adding CO2 into the bioreactor, that result in simultaneous formation of elemental sulphur and crystallisation of bicarbonate. The sulphur and. bicarbonate can then be separated off in a single step. This embodiment is particularly advantageous in special industrial plants producing spent caustic or similar unwanted by-product streams, such as refineries at remote locations, where water supply and liquid waste disposal are expensive or otherwise problematic.

A central step in the process of the invention is the provision of a solution containing sulphide ions, in particular a solution of sodium (hydrogen) sulphide. Depending on the origin of the sulphide, this sulphide-containing solution is produced by different pre- treatments.

First, if the sulphide originates as a solution, no further pre-treatment will be necessary other than proper dilution or concentration or adjustment of pH or temperature.

If the sulphide is produced as a solid or semi-solid, the (semi) solids are mixed with the supernatant from the carbonate recovering step, make-up water and pH control agents, resulting in a sulphide-containing solution.

In case of gaseous sulphide, i. e. hydrogen sulphide and/or carbonyl sulphide, the gas is scrubbed with an aqueous liquid at the appropriate pH, for example as described in WO 92/10270, resulting in a sulphide solution.

In a special embodiment, the sulphur-containing salts are oxidised sulphur salts, especially sulphite or sulphate or thiosulphate, which, prior to the treatment as described above, are reduced to sulphide according to the following equation: 2Na+ + so42-+ 8HF + 8 e~ o 2Na+ + S2-+ 4 H20 The eight electrons (e-) are supplied by hydrogen or another electron donor like ethanol.

In the case of hydrogen, the electrons are formed as follows: 4H2< 8H++8e~ Both reactions are preferably carried out using bacteria. The conversion can be carried out at a total cation concentration up to 8 M, preferably in the range of 2 to 4 M. The conversion can be carried out in the range from pH 1 to pH 13, though the preferred pH is from 5 to 9, especially from 6 to 8. The preferred temperature is in the range of 20- 35°C.

Here again, the starting salts may be provided as a solution, only requiring adjustment of concentration, temperature and./or pH, or as solids, requiring appropriate mixing and dissolution, or as a gas, in particular sulphur dioxide, requiring scrubbing with an aqueous liquid e. g. as described in WO 92/10270.

Bacteria capable of reducing sulphate and other oxidised sulphur species to sulphide include Desulforomonas sp. (mesophilic), Desulfotomaculum KT7 (thermophilic), the species Desulforolobus ambivalens, Acidianus infernus, Acidianus brierley, Stygiolobus <BR> <BR> azoricus (mesophilic), Thermoproteus neutrophilus, Thermoproteus tenax, Thermo- discus maritimus (thermophilic), Pyrobaculum islandicum, Pyrodictium occultum, Pyrodictium brockii (hyperthermophilic), and other species of the genera Desulfovibrio, Desulfotomaculum, Desulfosnonas, Desulfobulbus, Desulfobacter, Desulfococcus, Desulfonema, Desulfosarcina, Desulfobacterium and Desulforomas (mesophilic), and species of sulphur-reducing methanogenic bacteria such as from the genera Methano- coccus and Methanobacterium. It is preferred however, that the sulphate-reducing bacteria tolerating high salt concentrations. These include species of the genera Desulfovibrio, Desulfonatronovibrio and Desulfohalobium e. g. Desulfonatronovibrio hydrogenovorans (DSM 9292), Desulfovibrio halophilus (DSM 5663), Desulfovibrio salexigens (DSM 2638) en Desulfohalobium retbaense (DSM 5692). Electron donors to be used in the biological sulphate reduction can be hydrogen, carbon monoxide, alcohols, fatty acids and other readily degradable organic matter, as known in the art.

The sulphate reduction prior to the sulphide oxidation can e. g. be applied in case of concentrated sodium sulphate solutions issuing from reversed osmosis plants used for desulphurising sulphate-containing water. The reversed osmosis yields highly purified water, but it also produces a concentrated sulphate effluent. This effluent can be

advantageously treated using the process of the invention, and is then be disposed of or returned to the reversed osmosis.

Description of the figures Figure 1 shows the flow sheet for the conversion of a dissolved alkali sulphide. The stream originally containing sulphide passes through a sulphide-oxidising bioreactor for oxidising the sulphide to sulphur, a sulphur settler for separating off elemental sulphur, a bicarbonate crystalliser and a bicarbonate dryer. The resulting carbonate water can be reused in the bioreactor.

Figure 2 shows a similar flow sheet for solid material containing dissolvable sulphide salts. The installation of figure 1 is then preceded upstream by a solids dissolving unit and a separator for removing non-dissolved material.

The recycle in both flow sheets is optional. The bleed is required to let out surplus water and to bleed of accumulating salts. The location that is shown is an example. The bleed can be located anywhere in the liquid loop.

Figure 3 shows a flow sheet for the conversion of sulphate salts, comprising biological reduction as a prior step compared to the flow sheets of figures 1 and 2 carried out in an anaerobic sulphate-reducing bioreactor.

Exantl) le : Soda slag from a company that recycles car batteries by pyrometallurgical means was dissolved in water. The solution was clarified from particles by settling. It contained about 75 g/l of sodium (3 NI) and 45 g/1 of dissolved sulphide. It was fed together with a nutrient solution containing among other a nitrogen and a phosphorous source to a continuously operating 5 litre bioreactor at a temperature of 30°C containing T7lifo)- alkalivibrio strains comprising strain DSM 13738. A gas recycle over the bioreactor ensured mixing. Oxygen was added to the gas recycle in order to maintain the redox potential in solution to a value between-100 and-450 mV, preferably-360 to-430 mV measured with a platinum electrode against an Ag/AgCl reference electrode. The pH was measured with a glass electrode. It was controlled at a value between 9 and 12, in particular at about 10.5 through the injection of C02 gas in the gas recycle. Tlli (6) alkali- vibrio bacteria converted the dissolved sulphide to elemental sulphur. Effluent from the bioreactor was led through a settler where the sulphur was separated from the liquid.

The effluent of the settler was treated batch-wise with COx gas and the treated effluent was cooled to ambient temperature to precipitate the major part (>60%) of the original sodium as crystallitle NaEICO3 as described above leaving approx 1 M of sodium in solution.