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Effects of increasing leaching density on dissolution rate (5 replies and 1 comment)
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.
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.
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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.
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A mine is planning on increasing the leaching density from 40% solids to around 50% solids, with the aim of increasing the dissolution rates, although right now they are working at recoveries of 85%. what would be the effect on the kinetics of dissolution.