MILL EXPERIENCE OF CALCIUM CARBONATE SCALE FORMATION IN GREEN LIQUOR HANDLING SYSTEMS, 19PEERS
Calcium carbonate (CaCO3) often forms hard scale in the green liquor handling equipment in kraft pulp mills. In severe cases, scaling can lead to unscheduled equipment shutdowns for cleaning which can be costly. Scale is formed due to the extremely low solubility of CaCO3 in the highly alkaline green liquor environment that causes dissolved Ca2+ ions in the liquor to precipitate out during a process upset. This paper discusses the scale formation experience at four kraft pulp mills in Canada and Sweden and the results of systematic investigations conducted at these mills. The objective was to determine if there is any correlation between the severity of the scaling problem at each mill, and the design and operating conditions of its causticizing plant. Mill liquors, scale samples and process data (green liquor flow rate, density, TTA, causticity, sulfidity, temperature, suspended solids, etc.) were analyzed to determine the main operating parameters leading to scale formation. The investigation results help identify viable strategies for mills to adopt to minimize scale formation.
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