Alum treatment of caustic wash from chlorine bleached kraft pulp

Abstract

The purpose of this investigation was to continue the color reduction study of alum treatment of caustic wash from chlorine bleached kraft pulp, to determine if a double tray Dorr thickener could be used to settle the coagulated waste, and to determine if a centrifugal separator could be used to dewater the sludge from the Dorr underflow.

Approximately 5000 gallons of caustic wash from chlorine bleached kraft pulp was used for each phase of this investigation. The raw waste feed flow rate was varied from 1750 pounds per hour to 2499 Pounds per hour. The alum slurry flow rate was varied from 39 Pounds per hour to 60 pounds per hour. The hydrochloric acid flow rate remained constant at approximately five pounds per hour.

The raw waste was acidified with 38 percent hydrochloric acid to a pH of 6.0, and treated with alum sulfate slurry to coagulate the solids in the waste. The coagulated solids were concentrated in a double tray Dorr thickener. The percent of insoluble solids in the Dorr underflow varied from 0.65 to 1.35. The concentrated solids were fed to a Merco centrifugal separator where the insoluble solids were concentrated in approximately equal proportions in the Merco overflow and underflow.