Characterization of scales obtained during continuous nickel laterite pilot-plant leaching
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I. INTRODUCTION
THE majority of nickel is currently processed from sulfide ores. However, there is a trend toward the processing of nickel laterite ores, with a particular interest in the pressure acid leaching process. There are currently four nickel laterite pressure acid leaching processing plants, one in Cuba (Moa Bay) and the three plants being commissioned in Western Australia (Bulong, Cawse, and Murrin Murrin). Pressure acid leaching involves passing the slurried ore into segmented autoclaves, operating typically at 250 8C, and injecting acid into selected autoclave compartments. The slurry digests at temperature for approximately 1 hour, and then passes into flash tanks where it cools to approximately 100 8C. The solid leach residue then separates from the liquids in a countercurrent decantation thickener well. The nickel and cobalt present in the liquor are then precipitated either as sulfides (Murrin Murrin, Moa Bay), hydroxides (Cawse), or are extracted directly from the acidic leach liquor using solvent extraction (Bulong). The nickel sulfide or nickel hydroxide intermediates are dissolved and further refined by solvent extraction/electrowinning or, in the case of the Moa Bay plant, shipped off for further processing. The formation of supersaturated liquors in the pressure acid leaching compartments brings with it the possibility of material precipitating on the autoclave interior (scaling). Scale deposits in the Moa Bay digestion tanks at a rate of approximately 300 mm/year. Descaling, which results in a loss of production, may require 5 days per month.[1,2] Scaling has been examined in a continuous pilot-plant system using a Western Australian laterite in hypersaline water at 250 8C to 265 8C. These results indicate scale can deposit at approximately 150 mm/year.[3] However, it has yet to be determined whether scaling will be an issue in the autoclaves at the Western Australian plants.[17] This article B.I. WHITTINGTON, Research Scientist, is with AJ Parker CRC for Hydrometallurgy, CSIRO Minerals, Bentley, WA 6102, Australia. Manuscript submitted January 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
expands upon that by Czerny and Whittington[3] and describes a methodology for characterizing three different scales depositing within the autoclave during the continuous nickel laterite acid leaching runs (Table I). This article also compares the effect of digestion conditions on scale mineralogy by examining scales formed upon leaching a Western Australian laterite in hypersaline liquor and those formed upon leaching a tropical laterite in fresh water (Table II). Typical Western Australian ores consist of approximately 15 pct limonite ore (primarily goethite, FeOOH), 40 to 45 pct smectite clays (nontronite, Fe2 Si4O10(OH)2), and 40 pct saprolitic ore (serpentine group species, Mg3Si2O5(OH)4), while the tropical ore consists mainly of alumino goethite. For comparison, the Moa Bay and Cawse ores are limonite (goethitic), while the Bulong and Murrin Murrin ores are serpentine and nontronite rich.[4]
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