Scale formation in the digester during kraft pulping represents a great problem in pulp mills. Scaling reduces pulping control and efficiency, increasing energy costs and leading to cleaning breakdowns, with subsequent losses in productivity. The kraft process promotes CaCO3 scaling due to high calcium ion and carbonate concentrations, as well as high alkalinity and temperature levels, which increase the speed with which liquors reach a state of supersaturation. This work examines the action of diethylene triamine penta(methylene phosphonic acid) (DTPMPA), either alone or combined with commercial anti-scaling agents, as an inhibitor of calcium carbonate precipitation in the kraft pulping of Pinus taeda. The theoretical amount of calcium deposited in the digester was obtained by mass balance. Soluble calcium was stable throughout cooking when using the phosphonates alone or combined with anti-scaling agents. When adding only DTPMPA, calcium stays in the pulp, rather than forming deposits.

The addition of a phosphonated chelant (DTPMPA) at different points of a TCF bleaching sequence and its effect on pulp properties were studied in this work. An industrial Eucalyptus grandis kraft pulp was submitted to a counter-ion exchange (Ca2+ or Na+ form) and was then bleached using DTPMPA in the washing or in the bleaching stages of two distinct sequences: OOpP and OQOpP (20 pulps). The counter-ion exchange affected fibre length, as well as the handsheets bulk and air permeability (higher for Na+-based pulps) and handsheet tensile strength, brightness, skeletal density, and total porosity based on Hg porosimetry (higher for Ca2+-based pulps). The hydrogen peroxide consumption in Op and P stages achieved the lowest values when the chelant was distributed rather than applied in a separate Q stage. The addition of chelant in the P stage provides pulps with higher ISO brightness (>85%). The chelant effects were always more noticeable in Ca2+-based pulps.

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