You are not giving enough information: 

Does the slurry contain clay? if it contains clay the dosing could vary between 40 and 100g/t solids.
Is the mineral milled hard rock what is the D10 and D50? If the D10 is about 10 micron the dosing could be as low as 5g/t or no flocculant required.
You should dilute to 0.025% or lower for efficient flocculation.
The energy of mixing is important in flocculation efficiency.

For your mine and flocculant concentration, I would estimate that you would dose between 7 and 10 cubic meter per hour.

Important also to remember that the amount will also vary a bit from time to time with the nature of the feed...particle nature and size distribution . You should correlate roughly your operation (resulting nature of the feed) with the amount of flocculant. There is nothing better than test yourself with small mixers, cylinders and beaker to establish the relation between nature of the feed and flocculent concentration to add. I hope it helps.

It also depends on the flocculant and mineralogy. Cationic? Anionic? Do the minerals release cations or anions into the water that interfere? Is the flocculant dry or pre mixed with water? The only way to tell for sure is testing with your minerals and water.

With the mine being a deep level mine in the free state I would expect that it would behave similar to other deep level gold mines in the RSA. That would imply little clay and minimum CEC (cation exchange capacity) and EC (exchangeable cations) which in turn imply little alteration of the process water chemistry. Deep level gold ores typically require 5 to 15 g/t flocculant and almost always a commonly generic type flocculant. If the flocculant dilution is correct and the feed density not too high, then ask you flocculant supplier to do some settling tests. they normally do it for free. Kind regards.

The flocculant selection, the amount to add etc depends on mineral characteristics,, pulp density etc.
There is no alternative but to carry out tests. In fact in many cases the flocculant manufacturers, because of their data bank, may carry out some tests and guide also.
In order to check the suitability and to optimize the flocculant dosages ,the jar settling tests in lab scale needs to be done. Again the what underflow solids you need to maintain will also be the factor. The settling temperatures can also play a role.

Thank you all for your input. we managed to diagnose the problem i.e. it appeared that the concentration of our flocculant was insufficient and due to the change in season the temperature also had a negative impact on our thickeners.