PROCESS FOR PREPARING A BUFFER SOLUTION IN A PAPERMAKING PROCESS

摘要

The invention describes a process for preparing a buffer solution in a papermaking process wherein the buffer solution is used for controlling the alkalinity, pH and/or hardness of a process stream, e.g. an aqueous pulp suspension, in the papermaking process. In one embodiment of the invention the basic substance, sodium hydroxide, 1 and water 2 is fed together in a mixer 3 to get a diluted sodium hydroxide containing aqueous solution. The acidic substance, carbon dioxide, 4 in the gaseous phase is introduced in the dissolver 5. In the dissolver 5 the gaseous carbon dioxide is dissolved in a re-circulated buffer solution 6. The aqueous solution containing diluted sodium hydroxide is fed to the dissolver 5 too. The re-circulated buffer solution 6, which is withdrawn from the reactor 7 and pumped by the pump 8 into the dissolver 5, is buffered by sodium bicarbonate, which is formed in the reactor 7. Thereby the pH of the re-circulated buffer solution is according to the invention below 7.5 at all the time when the basic or acidic substances where mixed together. The re-circulated flow of the buffer solution 6 prevent a local increase of the pH above 7.5 due to the introduction of the sodium hydroxide containing aqueous solution and thereby the precipitation of calcium carbonate. The pH is always below 7.5 when carbon dioxide is introduced. The sodium bicarbonate containing buffer solution is created in the reactor 7 where bicarbonate ions are formed. The resulting bicarbonate ions containing buffer solution 9 is supplied to the main process stream, the aqueous pulp suspension, for controlling the alkalinity, pH and/or hardness.