In the production process of phosphoric acid, after the phosphate rock is reacted with concentrated sulfuric acid, two phases form: a solid one, which is the phosphogypsum, and a liquid one representing low-grade phosphoric acid. The latter is evaporated downstream to produce the commercial grade of the acid, giving sludge as a byproduct. Even if filtered, that sludge still contains considerable amount of phosphoric acid in addition to REEs. To process the sludge and recover both components
In all centrifuges, both inlets as well as the heavy phase outlet are closed to increase the separation efficiency, whereas the sludge directly flows to the rotor bypassing the high-shear annulus. Once sludge enters any stage, the largest portion of solids accumulate inside the rotor, while the separated liquid with some entrained solids proceeds and leave the device from light phase outlet. When separation efficiency starts significantly decreasing, this means that the unit reaches the saturation point, where rotor interior becomes full of solids. At that point, the system is shut down and the trapped solids are flushed out using recycled water.
The mass flowrate of sludge feed stream was 69 g min−1 with 32 wt % solids. At the outlet of the third stage, that content decreased to 1.6 wt % after 5 min of operation at 4000 rpm. Thereby, around 94 % of the solids containing REEs was recovered, and 98 % of the liquid was also retrieved as commercial grade phosphoric acid. If only REEs are of interest, one stage can be enough, since the highest content of REEs exists in the largest particles that separate first.
Compared with the traditional box type and tower type extraction equipment, the centrifugal extractor has the advantages of short stage retention time, rapid phase separation and wide flow ratio range; Liquid materials with different densities and viscosities can be met by changing weir plates and frequency conversion speed regulation, with strong adaptability.