Hi there Dimay

1. You are going to get a variance/variation on a daily basis between the two figures, however the error caused by this is lessened over time - so your leach recovery on a  daily basis may not be entirely accurate, over a week it will be more accurate and on a monthly basis it will be pretty darn close. I have worked on plants where the metallurgists have done a mass balance metal accounting, it can be done, you need to be able to tie the samples to the correct times and to be able to link back based on the volumetric flowrate through your circuit to link sample values. This in combination with variations in the true residence time (due to short circuiting, possible sanding in the leach etc.) is why the generally accepted method is to accept the daily variance. Unless if you have huge changes in head grade to your plant it shouldn't be too wrong - although of course it doesn't help in the immediate time frame to show if something goes wrong in the process!
2. Do your CCD thickeners work to wash the tails and reduce the cyanide in the tailings stream, with the water being returned to your process water system? If so, or if you have any other kind of tails thickener with water being returned immediately to the circuit, then yes, you probably have leaching in your milling circuit - I have seen some surveys return quite significant values for how much leaching is actually occurring in the milling circuit under these conditions, even without additional cyanide being added to the mills. This is easy to check - take a sample of your process water, assay for gold and cyanide (total, WAD and free), and do the same for the cyclone overflow solution. Any variation in gold value is gold leaching in the milling circuit. I have also seen significant leaching occurring in the CCD circuit after a gold leach - however, in this case the leach had a compromised retention time and significant short circuiting. Leaching can therefore occur in the thickener, but the extent thereof will depend on how well the upstream leach is operating. Again, you can do a survey to assay the solids and see over four or six hour periods and the course of a few days what the gold in the feed and underflow streams is doing.

Cheers

Colette

Dear Colette

Yes their aim was to wash the tail and reduce cyanide but due to water balance problem we are not using any wash water. Almost 10% of CIL tail water is returned back by the CCD everyday.

I have done test for pre-leach thickener over flow contain 1.77ppm of Au and for CCD over flow 0.042ppm Au.