Control of silica scaling during acid leaching of lateritic ore

In a method for inhibiting the formation of silica scale on flash-pot surfaces during the multi-stage flashing of sulfuric acid pressure leach slurry obtained following the continuous autoclave leaching of nickel lateritic ore at elevated temperature and pressure during which silica is dissolved, wherein said leach slurry is subjected to a high-temperature flash stage from a higher temperature down to about 190.degree. C. to 230.degree. C. such that the slurry at said flash temperature of about 190.degree. C. to 230.degree. C. is supersaturated in silica, and wherein said slurry is thereafter subjected to at least one additional flash to an intermediate-temperature flash stage in the range of about 120.degree. C. to 180.degree. C. at which silica precipitates and tends to form a scale accompanied by a decrease in said supersaturation, the improvement which comprises,

• recycling a substantial portion of the slurry to said intermediate-temperature stage following flashing to said intermediate-temperature flash stage while feeding a nickel laterite ore thereto to neutralize part of the acid in said intermediate-temperature stage and control the pH thereof to about 1 to 2,
• said portion recycled being sufficient to effect a dilution in the soluble silica concentration and thereby decrease the superesaturation in the slurry fed from the high temperature to said intermediate-temperature flash stage,
• and continuing said multi-stage flashing and said recycling of said flashed slurry to said intermediate-temperature flash stage,
• whereby silica scale formation at the intermediate-temperature flash stage is greatly inhibited.

In a method for inhibiting the formation of silica scale on flash-pot surfaces during the multi-stage flashing of sulfuric acid pressure leach slurry obtained following the continuous autoclave leaching of nickel lateritic ore at elevated temperature and pressure during which silica is dissolved, wherein said leach slurry is subjected to a high-temperature flash stage in the range of about 190.degree. C. to 230.degree. C. such that the slurry at said flash temperature is supersaturated in silica, and wherein said slurry is thereafter subjected to at least one additional flash at an intermediate-temperature flash stage in the range of about 120.degree. C. to 180.degree. C. at which silica precipitates and tends to form a scale accompanied by a decrease in said supersaturation, the improvement which comprises,

• recycling a substantial portion of the slurry to said intermediate-temperature stage following flashing to said intermediate-temperature flash stage,
• said portion recycled being sufficient to effect a dilution in soluble silica concentration of the slurry fed to said intermediate-temperature flash stage from the higher temperature stage,
• the amount of supersaturation after recycling to said intermediate-temperature stage being determined by the following formula:
• ##EQU8## wherein:
• S.sub.IT =supersaturation ratio at the intermediate temperature;
• F=the mass flow rate of the stream of supersaturated silica entering the high-temperature stage;
• S.sub.F =the supersaturation ratio of soluble silica in the stream being flashed from the high-temperature stage to the intermediate-temperature stage;
• R=the recirculation flow rate;
• and wherein F/R ranges from about 0.5 to 2,
• and continuing said multi-stage flashing and said recycling of said flashed slurry to said intermediate-temperature flash stage,
• whereby silica scale formation at the intermediate-temperature flash stage is greatly inhibited.