The Effect of Switchable Water Additives on Clay Settling

The recycling of process water from strip mining extractions is a very relevant task both industrially and environmentally. The sedimentation of fine tailings during such processes, however, can often require long periods of time and/or the addition of flocculants which make later water recycling difficult. We propose the use of switchable water additives as reversible flocculants for clay/water suspensions. Switchable water additives are compounds such as diamines that make it possible to reversibly control the ionic strength of an aqueous solution. Addition of CO2 to such an aqueous solution causes the ionic strength to rise dramatically, and the change is reversed upon removal of the CO2. These additives, while in the presence of CO2, promote the aggregation of clay tailings, reduce settling times, and greatly increase the clarity of the liberated water. The removal of CO2 from the liberated water regenerates a low ionic strength solution that does not promote clay aggregation and settling until CO2 is added again. Such reversible behavior would be useful in applications such as oil sands separations in which the recycled water must not promote aggregation. When added to kaolinite and montmorillonite clay suspensions, switchable water provided process waters of lower turbidity than those additives from inorganic salts or by CO2-treatment alone. When recollected, the switchable water supernatant was shown to be recyclable over three cycles for enhanced settling of kaolinite.