Patent ID: 6878357

Claim:
In a BAYER circuit including a preliminary agglomeration phase, a crystal growth phase and a classification phase, a process for controlling precipitation of alumina hydrate from a slurry resulting from introduction of recycled alumina trihydrate seed into an aluminate liquor, in which particle size quality of alumina hydrate produced in the circuit and circulating in feed tanks is monitored, comprising the steps of: a) a calibration step including: a1) measuring, versus time, of: percent of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X2 μm; and percent of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X1 μm; where X1 and X2 are predetermined particle sizes in microns and X1 is smaller than X2; and a2) determining a relationship R between percent finer than X1 and later changes in percent finer than X2 μm, and defining upper and lower trigger thresholds of percent finer than X1 μm which correspond to maximum permissible variations in percent finer than X2 μm; and b) controlling the circuit, comprising measuring percent finer than X2 μm and forming a correlation between percent finer than X2 μm and the particle size of hydrate produced by the circuit, measuring percent finer than X1 μm and updating the relationship R, and causing corrective action to the slurry at the beginning of precipitation when the measured value of percent finer than X1 μm reaches an updated trigger threshold, to bring the percent finer than X2 μm within the maximum permissible variation.