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switches. This should be done periodically so that the extractant phase surge tank volume can be adjusted as needed to maintain sufficient extractant phase volume to provide steady operation over extended periods, for example weeks, months and possibly years. The stirrers for the mixers were then powered up, adjusted and maintained at steady-state by the following procedures. 

All of the mixers were set between 700-1700 rotations per minute (rpm). The mixers needed at least 15 minutes to warm up, preferably 30 minutes. During this time the mixers were monitored and adjusted, usually decreasing the rotation rate in order to avoid excessive mixing. Excessive mixing is very undesirable; it can lead to problems such as spatter as well as the formation of fine emulsions that require longer phase coalescence time in the settlers. Although any type of stirring is sufficient enough to mix medium to low viscosity immiscible fluids, disk or fin type stirrer pumps are pre› ferred. They are both designed to pull the two fluids, aqueous and extractant phase, into the mixing compartment from the upstream mixer settlers. The shearing blades of the mixers generate micro droplets that create a very high interfacial surface area that is critical to fast contaminant extraction and strip kinetics. Higher mixing speeds accom› modate shorter residence time of the fluid in the mixer and compensate for extractant/aqueous ratios other than 1: 1. Although stirrer speeds that result in the mutual blending of only 20% of the two phases is sufficient, a blend of at least 80% is preferred and if optimum conditions can be achieved 95-100%. Excessive mixing is suitable but less preferred if the resultant emulsion formed requires mixing for long 

periods of time to disengage and break due to exceedingly fine droplet size. Mixing conditions preferred by the inven› tion is about 12-120 seconds, preferably 30-90 seconds, and optimally 45-seconds. The total hydraulic fluid residence time in the apparatus will be the sum total of the volumes of the individual operations of the apparatus, including mixers, settlers, pumps and surge capacity. Due to the low values and/or quantities of the contaminants present in the Acid Mine Drainage water purification process (Fe, Al, Se, Si, Mn, Zn, Ni, Co, Ca, SO$_{4 }$ - 2 ), and the very high flow rates of 10-10,000 gal/min (averaging about 500 gal/min, but often variable) of water, conventional metals extraction by LLX is not feasible because of the very large equipment and E/A ratio requirements that would be required for the large aqueous flows involved in water purification. Although the mixing conditions can be either extractant phase continuous or aqueous continuous, the latter is the more typical since special startup conditions are not needed to achieve it. 

After this initial loading of strip solutions and the warm up time for the mixers are both complete, the system was now ready for the charging of the Acid Mine Drainage feed water and (the acid mine drainage feed pump was set to 145 ml/min at system start-up) the extractant phase feed streams. The N-extractant (extraction circuit designed to extract +2 metal ions) feed flow rate at system start-up was set to 28 ml/min, and the M-extractant (extraction circuit designed to extract +3 metal ions) feed flow was set at 7.3 ml/min. The extractant surge tank for this process was a 6 L clear chemical resistant PVC tank. Other feed agents are listed in the LLX Test Condition Key in Table 5. 

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