Dredge, help, improvements and suggestions

Hi, I’m looking at re configuring the dredge I recently brought, it’s been well made by someone in the South Island previous to who I brought it from.
It runs a swimjoy pump on a 9 hp honda engine.

( my mistake it’s a gx 140 5.5 hp running a p180 equivalent pump.) Suction hose is 3 meters.

It’s clear from the great help I need to classify and build a wider box. Will email and thank those who have generously contributed personally.

I have 17 riffles 50mm x 250mm, with classification this may help keep the boxes open and stop clogging? (Insert some technical term here)
Mixing in expanded mesh may also help clearing and capture?

I’m chasing flood gold so is flaky and light. Maximum size to date is .3gm

I. Working my permit near Kumara, West Coast.

Problems, tends to block up the pipe unless running at 3/4 or above, resulting in jetting out over the box.

It’s got a 5 inch suction pipe on it which is probably over sized for the pump.

The box is 250mm wide by 1700mm long. It runs riffles and collects lots of rocks etc. It has about 100mm deep of water when running going through the box and the end of the box is submerged… the force of water going out seems to not cause any back pressure.
I tend to have better recovery with my hand sluice box.

Improvements, I am looking at or have done.
I put a grate on the nozzle to reduce the size of rock going through and blockages but it just means less production and more arm and hand work.

Would periodically stopping dredging and running full throttle to clear the suction hose upset the sluice bed if not picking up gravel?

When things open up again, I want to set up a classifying mesh or punch plate in the box, this will have a wave at the start to deflect any jetting due to blocked up suction hose.

Does anyone have experience with gold hog Matt’s or dream Matt in a dredge? I guess they would reduce the concentrates but imagine the rocks rolling would unsettle the bed holding the gold.

If I run it over 3/4 throttle I am concerned the light flood gold will float away.

Pros of the unit is the portability and width, can break down and carry into tight gorges…

Classification is probably the most urgent , with a longer and heavier dampening mat, in future will re engineer a wider box. It wouldnt take much to lift the box and engine.

If any one has some suggestions or input that will help that would be great.

New users can’t put up more than 1 photo sorry.

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Hi J.A., Without seeing the system in operation its hard to offer detailed advice.
But my initial comments would be, the sluice box is very narrow for a 5" dredge. My 6" runs a 560mm wide sluice, and my 4" a 400mm wide sluice.
If you want to recover fine gold you need to classify the material, it is the key to fine gold recovery. The following articles are a great reference on this:

Here’s a picture of my 6" setup I built. 3 stages:

  • High grade section at head of box where 90% of the gold gets caught.
  • Large nugget section (above 12mm) luckily where I dredge we get a few.
  • Fine gold section, ideally this would have dual classification but I was constrained by flare entry height.

Hey J.A

A couple of really important questions first-
Is the dredge powered by a powerjet or suction nozzle? If powerjet is it single or twin jets?
Does the dredge have a flare, or crash box or just straight into the sluice?
How long is the dredge hose?
Approx how high is the head of the box above the water when it is running?
What is the inlet and outlet size of your pump? Do you know how many Litres per minute and its head pressure are?
Does the sluice have a rubber dampener at the head of the sluice?

Looking at you sluice its way undersized for a 5 inch. For fine gold recovery as Dave Mccraken said you need to reduce the turbulence in the box and classify, classify and classify.

In order to get fine gold recovery on a box most people talk about the the importance of the sluice (which is very important) but thats only half the story. What you need for good fine gold recovery is get your self a well designed flare and rubber dampner

A flare allows the water to slow and for heavies to drop out of and ride the bottom of the flare so when they hit the sluice they go blip straight into the top of or head of the sluice .

A flare will also allow you to reduce the height of the column of water that is lifted above the water thereby increasing efficiency which translates into to increased suction.

A well designed flare will also allow greater extraction of the energy of the high pressure jets of water that create the venturi effect if your running a powerjet.- Translation- Greater efficiency= more suction

As for a rubber dampner- this creates a low pressure zone at the head of the sluice which causes heavies to fall out. Its other function is to break the surface tension of the water so any gold particles riding on top of the water (Similar when your panning fine gold in a pan and you see the gold floating on the water) drop out into the sluice.

Do yourself a favor- I repeat do not run Gold hog mats in a dredge. Biggest waste of money. Been there done that, and done back to back testing. Gold hog mats work well as cleanup mats or in environments that you can control a lot of variable like sluice pitch, water column height, classification, water velocity etc. Dredges run deep water with varying flows of water as material goes up the nozzle especially as you go up in dredge size. The ability to modify pitch on a dredge is limited.

For fine gold recovery in a sluice you really want to be running a over/under setup. The idea behind this is classified material is removed from sluice and treated in a lower turbulence environment.

As for Keenes 3 stage as alluded to above by another user- This is a form of an over/under. Works very well.They are alot heavier so you need to have the proper flotation. they also require a bit of playing around to get pitch right and know how to use the choke in them and ensure you have enough water flowing through the box.
Alot of people will say they tend to load up in the bottom stage. They are actually designed to do this as it acts as a giant fluid bed( but thats another story)

Another really good way to get really fine gold is running a sump setup but that starts getting up there in complexity and cost.

Once you come back with those answers to those questions above I can give you more specifics


Thanks Prich. Much appreciated, will reply in a couple of days when can load up some more photos and drawings. Great technical advise!

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Thanks Prospect Solutions! Nice work on your box, sharp engineering… I will reply with some more photos and drawings in a couple of days when can get measurements and up load. I want to calculate the surface area of my riffles and compare to expanded mesh… But as you have pointed out classification is key… I will try to work with what I have for now but may be best to widen out the box… The benefit of the narrow dredge is the creeks are narrow papa gorges. Photos to come… Thanks alot. J

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Im with you on the hog mat, I watched a picker work its way down my box , my theory is the shock wave caused by rocks hitting the rubber , make your heavies jump to the next riffle . like you say if your materials already classified like in a clean up situation , no worries , but great for lighting fires with tho

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Hi Webby.
Thanks! It looks like in a dredge the rocks action would unsettle and classify most heavys out the ass end. I’m curious about incorporating the dream matt into a classified section…
But if it isn’t broke dont fix it, looking to mix up riffles and expanded mesh over miners moss.
Cheers for the contribution.

Do you make these to sluice boxes to sell by chance?

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Hi Kman, drop me a line at sam@prospectsolutions.co.nz let me know what you require and I will try and get something priced up for you, cheers

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Hi J.A
While I’m just a home build novice in this field I did find those same issues as you have explained. I was just dredging for flood gold being flakes and the odd small picker. To improve the free flow I replaced all riffles with raised expanded steel with ribbed carpet underneath. (this carpet is used in jet boats) and widened the box to 300mm & I made the box slope adjustable. Gravel entry is by crash box which surprisingly works very well in spreading the gravel evenly down the box. This setup works very well for my 2’’ nozzle.
All the larger stones just roll down the box. The flakes are always found within 200mm of the falling point.