L'invention est relative à un procédé de traitement d'un effluent gazeux pauvre en anhydride sulfureux, en vue de l'obtention d'une solution concentrée de bisulfite d'ammonium ou de métal alcalin ou alcalino-terreux, dans lequel on soumet ledit effluent à une absorption dans une solution de sulfite et bisulfite d'ammonium ou de métal alcalin ou alcalino-terreux de pH compris entre 5 et 7, puis on met en contact au moins une partie de la solution obtenue avec un gaz riche en anhydride sulfureux.

Application au traitement des effluents gazeux issus des unités de production d'acide sulfurique.

@ There is provided a method for producing compositions which can be used for preparing sodium dithionite, in which sulphur dioxide is brought into contact with a recirculating aqueous solution of sodium hydroxide and previously absorbed sulphur dioxide to a ratio of sulphur dioxide and the sum of sulphur dioxide and sodium oxide of 0.61-0.66, whereafter the resultant mixture is pressurized and liquid sulphur dioxide introduced therein to a ratio between sulphur dioxide and the sum of sulphur dioxide and sodium oxide of 0.75-0.81, and the water content is adjusted to a concentration of sulphur dioxide + sodium oxide of 10-13 percent by weight. The invention also relates to the use of the prepared composition for the manufacture of dithionite.

The invention relates to a process for preparing potassium thiosulfate, potassium sulfite or potassium bisulfite comprising the following steps:

Step (1a): providing a potassium hydroxide or potassium carbonate solution for neutralizing acid forming components such as dissolving SO₂ or H₂S;

Step (1b): providing an SO₂ contacting solution, containing at least some potassium sulfite or potassium bisulfite or potassium thiosulfate;

Step (2): providing SO₂ gas;

Step (3): reacting these to absorb the SO₂ gas and to form an intermediate reaction mixture comprising potassium sulfite, or potassium bisulfite or a mixture thereof, and optionally recovering the potassium sulfite, or potassium bisulfite or a mixture thereof, and/or optionally using steps 4 and 5;

Step (4): adding sulfur or sulfide containing compound containing sulfur having the oxidation state of 0, -2 or of between 0 and -2 to the reaction mixture and optionally potassium hydroxide or potassium carbonate, and reacting the mixture under suitable conditions to form potassium thiosulfate; and

Step (5): recovering the potassium thiosulfate, and optionally concentrating the potassium thiosulfate.

@ There is provided a method for producing compositions which can be used for preparing sodium dithionite, in which sulphur dioxide is brought into contact with a recirculating aqueous solution of sodium hydroxide and previously absorbed sulphur dioxide to a ratio of sulphur dioxide and the sum of sulphur dioxide and sodium oxide of 0.61-0.66, whereafter the resultant mixture is pressurized and liquid sulphur dioxide introduced therein to a ratio between sulphur dioxide and the sum of sulphur dioxide and sodium oxide of 0.75-0.81, and the water content is adjusted to a concentration of sulphur dioxide + sodium oxide of 10-13 percent by weight. The invention also relates to the use of the prepared composition for the manufacture of dithionite.