Hi Paulo

In my experience low flowrate in Vat leach tanks is always caused by poor percolation through the sand being leached and is almost always a function of the coarseness of the sand. Too many fine particles (50 microns or less) blocks the pores between the coarse particles.

A good vat leach tank has a 100-200 mm thick layer of bricks or brick sized rocks at the bottom, packed a small distance apart (10-50mm) so that channels for solution flow is created. Place a layer of hesian cloth or plastic bags with small holes all over on top of the bricks and then pour coarse river sand on the cloth layer to a depth of 200-300 mm. You can place another layer of cloth /plastic bags on top, but I never bothered with this last layer. You can now place your gold bearing sand on top of the river sand. The depth of gold bearing sand should not exceed 1-1.2m deep since the rate of percolation drops of noticably if you increase the depth beyond this value.