Two samples of TI2S - contg. suspensions were taken from wastewater of H2SO4 plant, one of the stream before, and the other after the TI removal. From the second sample, the TI2S was pptd. in the lab. The grain size compn. of suspensions was detd. by using the sedimentation balance and a laser diffractometer. The TI2S particles had a form of flocs very sensible to hydronamics forces. A lamella settler was used to sep. the sediment with a high efficiency (about 99%). Numerical methods were used to design the settlers.