Assuming that you are running the cutter head in double double cut mode at bottom then you will have feed material containing high levels of heavy minerals.
Double cut mode means that you pump and dump all of the material above 1 metre up from bedrock.
Double double cut mode means that when you have cleared all of the material above the 1 metre level you treat all of the remaining material down to bedrock in the next passes and then come back to try and get any bedrock material which is soft enough to be excavated.
The bedrock is usually the highest grade material and considerable gold values can be lost by running it with the heavy material feeds from just above the bedrock.
You usually have a diver checking the bedrock to locate softer areas for dredging.
The feed to processing will now contain a lot of heavy, usually iron, sands.
Despite the promises of the sellers of all centrifugal equipment this type of equipment cannot treat poorly classified (say < 10mm screen) heavy rich material, it chokes on the heavies and if set to clear the heavies it will lose fine gold.
The more classification you do the better they will perform but they will all have the problem with the heavy material at the bottom sizing.
A jig will take the same sized feed as above and get out a concentrate with most of the gold down to 75 microns in the con.
Losses of finer gold start at this sizing, most operators will run poorly sized and inconsistently mixed feeds so that you get slugs of feed mixed with times of just water, this tends to make a mess of finer recoveries.
Your sluice will get most of the gold > 100 microns if set up and run properly, if run as per the TV dramas you will be lucky to get any gold below 300 microns.
Those are the main methods of primary recovery.
If you have cons from any of the above above primary recovery systems then you need to clean them up for smelting.
The one thing that the TV series does get right is that the best way to clean your cons up is with a small Wilfley or similar table.
Set up and run properly you will get a very clean con which, if the residual iron is removed with a magnet, can be direct smelted without using any flux.
Deano