New Zealand Gold Prospecting & Metal Detecting Forums Archive

This game of gold hunting , mining, fossicking, panning, sluicing, detecting, dredging etc & what ever else you might want to call it sometimes. Never ceases to enthral & amaze me on all fronts. I got hooked back in '93 & have pretty much just done it all on my own, as in by myself. Made most of my own gear from the top of my head & with a bit Kiwi ingenuity. Thinking I was pretty clever & was the only person to have done so. Well so I thought, until when I hit the net & dicovered that there is nothing new under the sun. It was this passion of gold mining that got me onto & into computers in the first place. Otherwise I would still be computer illiterate. That there were plans out there, on all my ideas that I thought I had invented & was going to make my fortune on. And they were all available to any body for nothing. Like the gravity dredge.
Back in '93 I was sluice boxing away, with that box I have now made into my highbanker/banjo, shoveling gravel thru, as you do.



Wishing I had a suction dredge that would be able to suck up every morsel off the bed rock at the bottom of the hole I was getting deeper down into, & the bottom wasnt far away. I was up to my chest in waders & the water bloody cold. All snow melt. I knew with the shovel I wouldnt be able to get everything off the bottom. Plus where I was working was in a creek that was carving its way thru schist/quartz bedrock dropping steeply down a series of waterfalls. At the base of each waterfall is a rock pool carved out over the years from the waters continual impact from the falls.





You will notice the sluice box set up at the top of the waterfall in the middle of the below pics where I was shoveling material out of the hole into the box. Then realised how deep the hole was getting. I used the pipe to try & drop the water level in the hole but it wasnt big enough to do that. Then the gravity dredge idea was born. These few pics are of my first gravity dredge attempt.







The rock pools being where the gold is hanging out. So I got the old grey matter burning. I didnt want to go to the expence of a pump & motor or the noise of one that would give me away as to what I was up to, & I didnt want that attention or my spot discovered. As it is in a public fossicking area & the use of hand tools only NO motorised equipment allowed. So the idea that I thought I had dreamed up was a dredge set up that runs on the same simple principle as siphoning petrol from a higher container to a lower container with gravity doing the work. Only I used a 2" pipe with some green flexi hose at the top end, like a vacuum cleaner hose & pipe set up. This I use at the top where the hole is that im wanting to suck out.





The 2" pipe runs from there down hill to where my home made sluicebox is with home made headerbox that has an old fridge tray in it for classifing the material to less than 20mm before it carries on thru the sluicebox. Much like a highbanker set up. So any material larger than 20mm just falls out the front. The bonus of pre-classifing the material is you dont need to keep on coming down & check for possible rock blockages in the headerbox or riffles. Plus it improves the fine gold recovery as you dont need as stronger water flow to wash the gravel thru the box as you would to wash thru rocks of say 2".



To get the whole thing running, I block the bottom end down in the sluicebox with a tennis ball. Go up to the top end & fill the pipe from the sluicebox all the way up to the top flexi hose with water. Make sure the top flexi pipe hose end is in the water, Usualy jammed under a rock or in a crevice. Then I race back down to the sluicebox & pull out the tennis ball, & hay-presto you have a dredge operating with no noise & no operating cost factor.





Then its just a matter of going up top & vacuuming out the rock pool. You have to manhandle large rocks out of the way plus any other cobbles that wont go up the nozzel. Sometimes it involves ropes & winches to move the bigger rocks but thats all part of the fun. I made an underwater viewfinder out of a 300mm length of 150mm diameter pvc stomwater pipe with some glass siliconed in the end. I can hold that in one hand & view what im doing in the water with the suction pipe in the other. You can sometimes see the bigger bits of gold as you suck em up.



Gold found on this attempt





Happy hunting

JW :)

Edited by user Monday, 26 April 2010 5:57:38 AM(UTC)  | Reason: Not specified

Interesting, never come across a gravity dredge before - awesome idea. I would think DOC couldn't get too upset about this one?!

Hi there, When I first set this idea up my aim was to run a 2" suction nozzle. But I very soon found out that that was not going to give any where near the pressure to run a nozzle. You would need a lot more fall & bigger pipework, like they did in the old days to run their hydrolic monitors, to get the required squirt to run a nozzle or power jet. I just ended up using the syphone suction to do all the work. Just like syphoning petrol but on a bigger scale & longer pipe work.
I have done the same thing with a 4" pipe. Works a treat.





JW

excellent stuff JW
Heres a wee piece I saw on youtube last year.
http://www.youtube.com/watch?v=esN2JFS0QYc

Yep might thoughts to JW, saw the clip last year any and back pressure would most likely kill the small amount of suction.
Cheers Steve.

This layman liked the dea of the layflat hose. I guess this wa referring to the type of foldable hose that the firemen use. Folding/ rolling it up flate, you'd be able to backpack in a lot more hose... and hence increase the pressure/ suction?

Look at the lack of water he is dealing with. I think its a great video to show the principals of gravity dredging
why so negative all the time jw.

Hello KiwiJw,and others too.
Been there, done that senario years ago.Works really well if you can get the fall.
I used approx 10 mtrs of 30 cm poly-ethylene "stiff" water hose, dangled over a 6 mtr water fall into a large bucket filled with larger stones, the falls had large rugged pools at its upper "head" end, which I classified by raking all the rocks larger than say 20cm with my "specially spaced" rake head.
Note: You dont need much fall to create a powerful suction (stick your finger in it and you have to lift it out of the water and some how give it access to suck a bit of air in so as to break the suction and get your finer free.)Suction is a real powerfull force!

Regards---CRiticol.

i have seen on a you tube clip, but cant find it any more that showed a gravity dredge with the sluice box at the top end connected to the nossel via a tube of about 3m. the main body of the pipe was creating a vacume in the nossel, but somehow diverting to a sluice close to the top end. ie no sluice at the bottom end of what may be 30m of pipe. ive tried to get my head around it but am stuck. is anyone smart enough to solve this? or am i mistaken

no not confusing at all, but if the sluice could be close to the nossel less gold would get trapped in pipe contections and clean up would be alot easier.