This is usually dependent on the viscosity of the pulp.  Typically, "fresh" ores (no clay) give similar leach extractions and kinetics at 50% solids (testwork from a recent study indicated exactly that).  However, if the slurry thickens appreciably at 50% solids the oxygen transfer may be impeded, plus, you may see activated carbon floating to the surface which is not a good thing.  Plots of pulp density versus apparent viscosity typically see an inflection point around 45% solids but the degree of inflection (upwards) depends on the amount of positively charged slimes particles in the feed (minus 10 microns).  This isn't always the case but is a reasonable guide.  Testwork at bench scale is always recommended before making big changes in the plant that can cost the owner a lot of revenue.  A pair of simple agitated leach or bottle rolls at 40 and 50% solids under identical conditions, taking timed samples at set intervals (2, 4, 8 and 24 hrs), should provide peace of mind.  It wouldn't hurt to measure dissolved oxygen as well if you have such a device on site but the proof is in the extraction rates but it is not essential. 

An interesting analogue test for pulp viscosity you can use is called a Marsh Funnel test.  It was developed for drilling muds in the oil industry but it is also used by labs to establish if a sample's pulp density is adequate for IsaMill testwork.    Here's a link to a test method that you can easily do on site if you have a steep walled funnel. 

http://hamdon.net/wp-content/uploads/2015/02/Marsh-Funnel-Instruction-Manual.pdf

This is also useful:     https://en.wikipedia.org/wiki/Marsh_funnel

To work out the effective viscosity the following formula is used:

μ = ρ (t - 25)

where μ = effective viscosity in centipoise
ρ = slurry density in g/cm³
t = quart funnel time in seconds

For a typical gold ore of 40% solids w/w and 2.70 SG, the pulp density is 1.34 g/cm3.  With a proper Marsh funnel, draining in less than 40 seconds is usually indicative of a slurry that is fine for processing.  In this case:

μ = 1.34(40-25) = 20.1 cP

It doesn't matter that much if you don't have the right funnel as what you are really after is a comparative measurement.  If you measure the effective viscosity for your current operating slurry using the above method and do the same for 50% solids 1.46 g/cm3 density), you can determine if there is a potential problem.  If the number ends up being much higher (20 to 30% higher) you may have a problem.       

Let me know how you get on!

thank you for your comment, there are planning to use a reagent called, which will allow them to leach at high densities without compromising on the recovery. i think i will do some tests, to see how it affects dissolution and carbon settling. will also check out the links you sent me.

Also see https://www.911metallurgist.com/cyanidation/slurry-density-for-agitated-leach/

thank you very much. will post some testwork i did after i finish typing it.

FINISHED LAST WEEK 

Leach test at different densities Methodology

•  Bulk samples of thickener underflow was used for the test work
•  All samples were done in triplicate
•  Pulp was made up to densities of 45%, 50% and 55% for samples A, B and C respectively.
•  Sodium cyanide, caustic soda and hydrogen peroxide were added at a consumption of 1.3kg/t, kg/t
  and 0.6kg/t respectively.
•  All samples were bottle rolled for hours
•  pH was at an average 9.12
•  Solution samples were taken at hourly intervals for the first four hours, then 8hrs and .
•  Head grade for the bulk samples was assayed.
•  Tails at the end of the bottle rolls were thoroughly water washed and reassayed.