Presentation to the PAPTAC Bleaching Committee

October 17, 2000

Irene M. Yaraskavitch

Dow Chemical Canada Inc.

· unscheduled shutdowns for clean out

· plugging of washer wires ® reduces

flow ® poorer washing ® poorer

pulp quality

· flow/production reduction

· contamination of pulp

· corrosion ® by deposits and by acid

cleaning and boilouts

Two perspectives:

· thermodynamic - useful for chemical &

non-chemical process modifications

· kinetic - useful in identifying scale

prevention strategies

· solubility limit/supersaturation

· temperature, pH, chemical composition

· common ion effect

· ion-ion interactions - ion pairing, complexation

· driving forces for precipitation

· nucleation

· crystal growth

· phase transitions

· agglomeration

· adsorption/deposition

· antiscalents e.g. polyphosphates, phosphate

esters, phosphonates, lignin derivatives,

carboxylate polymers

· precipitation threshold inhibition

· dispersion

· crystal distortion/modification

"Mechanism and Chemical Control of CaCO3

Scaling in the Kraft Process", S.J. Severton, P.Y.

Duggirala, P.W. Carter, P.E. Reed, Tappi Journal,

Vol. 82, No. 6, 167 - 174 (June 1999).

"Inorganic Scale Control in Today’s Pulp Mills",

C.S. Greer, PIMA’S Papermaker, Vol. 81, No. 6,

86 - 87 (June 1999).

"Water-Formed Scale Deposits", Jack C. Cowan,

Donald J. Weintritt, Petroleum Associates of

Lafayette, Incorporated, Gulf Publishing Company.