Methods for recovery and re-use of additive chemicals during CO2 mineralisation

This paper presents options for the recovery and recirculation of ammonium sulphate (AS) and/or ammonium bisulphate (ABS) additive in a process for mineral carbonation referred to as the angstrom A route. The process uses magnesium silicate rock (serpentinite) reacting with AS and/or ABS in order to extract magnesium which subsequently will be carbonated. A challenge still is to find ways to recover the chemicals in an efficient way and this paper addresses options for this. The concepts and possible implementation for reverse osmosis separation, precipitation by the addition of alcohol, multiple-step extraction and ion selective membranes/ion exchange resins are presented. It was shown that reverse osmosis membranes will need high pressures in order to achieve sufficient separation. Addition of alcohol (ethanol) will make it possible to precipitate AS, magnesium and iron sulphates. This enables some separation of AS from ABS since ABS remains dissolved in the solution, but subsequent separation and recovery of ethanol from water is in turn required. Multiple-step extraction was tested with a procedure containing both thermal and wet extraction. It was shown, however, that precipitation of some species resulted rather than extraction, which has to be further investigated. The multiple-step extraction procedure included different ratios of rock: AS and/or ABS; it was shown that AS plays an important role in the thermal extraction even if ABS is a more aggressive chemical. The option to apply ion selective membranes for the recovery of ABS is part of ongoing work: first results are promising, as presented here.