Process for regenerating scrubbing solutions

A process for removing gaseous sulfur oxides from a gas stream by contacting the gas stream with a circulating aqueous scrubbing solution containing sodium sulfite, sodium bisulfite and sodium sulfate so that the sulfur oxides react with the sodium sulfite in the scrubbing solution to increase the sodium bisulfite content, said process comprising the steps of:

• monitoring the density of the scrubbing solution;
• withdrawing a portion of the circulation scrubbing solution in response to the increase in density of the scrubbing solution to a storage tank;
• when the withdrawn portion of the scrubbing solution in the storage tank reaches a prescribed starting level, transferring the scrubbing solution from the storage tank to a reactor vessel at a substantially constant flow rate;
• introducing calcium oxide in the form of pebble lime into the scrubbing solution in the reactor vessel for reaction with the scrubbing solution;
• repeatedly impinging substantially all of the pebble lime and scrubbing solution in the reactor vessel against an impingement surface in the reactor vessel to mechanically remove any film of calcium sulfite formed on the surface of the pebble lime as the solution reacts with the calcium oxide in the pebble lime so that substantially all of the pebble lime reacts with the solution to form an aqueous slurry containing increased sodium sulfite content, solid calcium sulfite and the unreacted residue of the pebble lime whereby the settling rate of the slurry is greatly enhanced;
• withdrawing the aqueous slurry from the reactor vessel at substantially the same flow rate as the scrubbing solution is transferred to the reactor vessel;
• forcing the withdrawn aqueous slurry through a plate type settler so that the solid calcium sulfite in the aqueous slurry settles by gravity to produce a concentrated slurry at the lower end of the settler and a substantially solids free aqueous solution at the upper end of the settler;
• withdrawing the solids free aqueous solution from the upper end of the settler as an overflow;
• withdrawing the concentrated slurry from the lower end of the settler as an underflow;
• filtering the concentrated slurry in a rotary vacuum filter and recovering the substantially solids free aqueous solution produced as a filtrate;
• combining the substantially solids free aqueous solution withdrawn from the settler with the substantially solids free aqueous solution from the rotary vacuum filter to produce a regenerated scrubbing solution with a reduced sodium bisulfate concentration;
• and
• returning the regenerated scrubbing solution to the circulating scrubbing solution.