The use of ultrasound to enhance crystallization of minerals from concentrated saline effluent

Many effluent treatment processes result in a concentrated brine solution that requires disposal. Bulk crystallisation of the salt can facilitate this disposal, by providing a solid that can be more readily transported. In this presentation, we consider the use of ultrasound to assist in initiating the crystallisation of salt from saline effluent sourced from the Australian dairy industry. The effluent has already been concentrated to 22 or 30 wt% total solids, these being principally sodium chloride, but with some calcium phosphate and lactose. The precipitation follows reverse solubility behaviour with respect to temperature, with the shortest induction times at the highest temperature. This reflects the low solubility of the calcium phosphate salts, which precipitate first. Ultrasound can significantly reduce the time required to induce crystallisation. The use of a higher power produces a greater reduction in crystallisation time, relative to a lower power at the same energy density. Further, the use of two pulses of ultrasound during the induction period, also shortens this induction period, relative to the same energy application as a single pulse. Indeed, using a double pulse reduces the induction time to around 30 minutes, relative to 2.6 hours without ultrasound. Ultrasound also influences the crystal morphology, with needle like crystals only appearing when this is utilised. However, the ultrasound cannot improve the crystal yield, which is limited by the relative humidity of the atmosphere above the crystallising