The chlorine dioxide production process uses sodium chlorate, methanol and sulfuric acid as reagents. Sodium chlorate reacts with methanol in a solution of sulfuric acid generating chlorine dioxide gas.
Chlorine dioxide gas is cooled and sent to an absorption tower, where a chlorine dioxide solution is produced that is stored in ponds, to then be used in the bleaching process.
In the chlorine dioxide generator, a secondary stream is produced that is sent to a rotary filter. On the outside of the rotary filter, a solid is mechanically removed, which corresponds to the acid salt of sodium sesquisulfate, which is entrained with water at 60°C to subsequently fall into a tank where it is neutralized with sodium sulfate soda, a product that is sent to the liquor circuit, joining the black liquor, prior to its burning in the Recovery Boiler.
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Crude Tall Oil (CTO) is obtained from the soap present in the weak black liquor coming from the fiber area, through a reaction with sulfuric acid. The sulfuric acid consumption is normally about 200 kg/ton of CTO, depending on the quality of the soap. This represents the majority of the use of sulfur in a modern kraft pulp plant.
When reacting soap with sulfuric acid, the pH of the solution decreases about 12 to below 8. At this pH level, the acidified CTO is separated into two streams using a centrifuge: Crude Tall Oil (lighter phase) and brine (mainly sodium sulphate and lignin containing hydrogen sulfide).
The brine is returned to the recovery circuit before the neutralization of sulfide with white liquor. The CTO is stored to then be sent as fuel to the Lime Kiln or well to be sold.