According to your comments, there is a wrong design. Although, you have not mentioned anything about the tank diameter. It is necessary to point out that there is relationship between the diameter and height. Typically, the height to tank diameter ratio ranges from 1 to 1.4. In some cases, much higher ratios can be considered if the leaching plant layout must be optimized. It is known that an appropriate height may create higher hydrostatic pressure at the bottom and improves the leaching kinetics. Also, when the ratios are as high as 1.1, a single impeller is enough the maintain the solids in suspension. With higher ratios than 1.1, two impellers should be considered in the design.

The impeller to tank diameter ratio ranges from 0.3 to 0.5. Other point to consider is the impeller location, it should be placed at a distance from the bottom similar to the impeller diameter. In the cyanidation tanks, when the air is fed down the shaft of the agitator and exiting the bottom of the shaft at a point below the impeller, the gold dissolution is improved because there is a good air dispersion. 

Adimulu,

You should get your agitators redesigned and changed-out ASAP. You could try running them backwards until your new ones are installed. This MAY help to reduce settlement. Any air added though the shaft will help with leach kintetics but pure oxygen is the best. Segregation should not be a problem as long as the agitation is adequate.

I can assist with these issues if you wish.

Cheers,

Rob

Adimulu, also note that a too high air flow under the agitator could and will lead to the agitator's flooding when air is basically preventing blades from moving the liquid. So increasing the airflow through the shaft could help with the suspension to some extent. But after some point it will only make the settling situation worse.

BR, Peter